Mobile Internet and Cellphones Throughout The Americas

Wireless and USB Modem
There is plentiful wireless connectivity throughout the Americas. Some countries offer free wireless in their town squares. We have a USB wifi booster/extender ("BearExtender" for $50) that we use to pickup weak signals at campgrounds or around town. It works so well, we can't say enough good things about it. With it, we use a free software program called "Virtual Router" for Windows that turns the computer into a wireless router. That way, we can share the amplified wireless connection between computers and our tablet.

In about half the countries so far, we have bought a prepaid USB modem (with a SIM card included). The rest of the countries we simply bought a SIM card (see next paragraph). The USB modem works with our laptops to access the internet via cell towers. On average, a prepaid USB modem costs $20 - $30 and comes with about 1 month of free data. If you're in the country longer than your initial package expires, you can buy additional internet packages (like $30 for 2GB/month) or service based on time (like $30/month for unlimited usage). It depends on the carrier and what their prepaid data plan offerings are. Our experience is that you can buy a prepaid modem in any of the bigger cities.

Another option is to buy an unlocked USB modem or smartphone/tablet in the USA. Then you would need to buy a SIM card in each country and setup your modem for each carrier. In Mexico and Central America, we found that the prepaid USB modems were cheap, and they came included with data/airtime, so it wasn't worth the hassle of setting up our own modem. If you are setting up your own device, this website has lots of information about settings: http://wiki.apnchanger.org

Phone
We swap the SIM card out of our prepaid modem and put it in our old unlocked cellphone for making phone calls (see the note at the bottom about unlocking USA phones). You can buy prepaid SIM cards in all countries for pretty cheap (~$3). Some carriers will give you the SIM card for free as long as you buy minutes or data immediately. Make sure you are given the phone number associated with the SIM when you purchase it.

"Recharging" or Adding Airtime
You can definitely add money/airtime anywhere - I'm talkin' in the backyard of Mama Maria's Fried Chicken Tienda in the middle of nowhere. To load your SIM with money/airtime, use the phone number of your SIM anywhere that advertises "recarga" for your carrier (or buy a recharge card to use at your leisure). Most chain grocery stores offer recharging ("Yo necesito recargar mi celular"). For internet usage, you then apply that money by signing up for an internet package. I don't really understand why it has to be a 2-step process. Some countries allow you to do everything online and some countries require you to put your SIM card into a phone and send text messages to buy and/or signup for stuff. When you buy a modem/SIM, you can ask the person behind the counter to set it up for you the first time. Bring your phone or computer with you, and make sure everything works before you walk out the door.

Per Country Information
In general, cell towers seem to exist anywhere there is a population. In Guatemala, we had coverage anywhere and everywhere (see below) . The trick is making sure you have the carrier that exists in the remote towns you care about. If you're in a bigger population-center, all of the carriers will have lots of towers.  But for example, when I knew we were going to Pavones, Costa Rica or Santa Catalina, Panama, I did a web search to try and figure out which cell towers were in town before we committed to a carrier (Kolbi and Claro). For Pavones, I ended up emailing a hotel and asking them which cell tower was in town.

Below, I list the carriers we used and where we bought modems/SIM cards.

Mexico carrier: Telcel
Telcel worked well anywhere that we imagined you might find a cell tower. If there isn't a population of people, there isn't going to be coverage (i.e. remote Baja).
Purchased in San Jose del Cabo at a Telcel store in a big shopping mall, Baja 2012

Guatemala carrier: Tigo (the fastest and best anywhere!)
We still talk about our internet coverage in Guatemala.  It was insanely fast and accessible IN THE MIDDLE OF NOWHERE! We never lost coverage and we drove to some seemingly remote areas. The bonus!? Read below...
Purchased near Las Flores at an electronics/cellphone store in a big shopping mall, 2013

El Salvador: Tigo (from Guatemala)
I don't know if this was a glich in their system, but we crossed into El Salvador and the Tigo Guatemala modem/SIM worked the whole time without having to pay for additional minutes.

Nicaragua: Claro
Purchased in Esteli at an official Claro store on the town square, 2013

Costa Rica: Kolbi with a Claro modem
Kolbi used to have a monopoly on the cellphone service in CR (used to be called ICE). But when we were there, they didn't have their own prepad USB modem for sale. The ICE office directed us to an internet cafe that is a Claro and Kolbi dealer. They sold us an unlocked Claro modem (Huawei E303s-3). We then had to buy the Kolbi SIM card and set it up in the Claro modem. The bonus is that we have an unlocked modem that we were able to us in Panama and Columbia, see below.
Purchased in San Isidro de El General at an internet cafe selling Claro and Kolbi service, 2013

Panama: Claro
We used our unlocked Claro modem from Costa Rica and got a free SIM card at a Claro store (as long as we loaded it with $5). We only paid for 2G speeds ($9 for 15 days of unlimited service) but you can pay twice the price to get 3G/4G. The only thing we had to do when we set it up, was change the APN to "web.claro.com.pa" through the connection software included with the modem (found via a google search "panama claro apn").
Purchased in David at the Claro store in the big shopping mall at the turn to Boquete, 2013

Columbia: Claro/Comcel
We used our unlocked Claro modem from Costa Rica. The SIM was ~$1.50. The weird thing about the setup is that we used all of the settings that were pre-programmed into the Claro connection software for Costa Rica. The apn is "ba.amx".  There's awesome info at wiki.apnchanger.org for Columbia.
Purchased SIM card at a small store in Cartegena (you can buy SIM cards everywhere!), 2013

Ecuador: Claro
We used our unlocked Claro modem from Costa Rica.  Ecuador was the first country that required an activated SIM for data use (internet movil). We made the mistake of buying an unactivated, regular SIM first - the only internet packages we could purchase cost $1 for 10 MB of data. We found a Claro store where they activated our current SIM specifically for modem use, after which we could purchase data packages for a much more reasonable price (still quite high compared to the other countries thus far). You can purchase a "data" SIM with 500 MB of data for $20 at official Claro stores - ask how to add data after your 500 MB expires. The apn that worked best for us: "internet.claro.com.ec".  Purchased SIM card at a small store right near the border but this was not a data SIM. It worked for emergency use at $1/10MB. We had to have it activated at an official Claro store at a mall near Ibarra, 2013

Peru: Claro
We used our unlocked Claro modem from Costa Rica. Peru is similar to Ecuador in that the SIM card must be activated to be used with a modem.
Purchased SIM card at the official Claro store in the town square of Tumbes (near the border with Ecuador). Passport was required.

Chile: Entel
We tried to use our unlocked Claro modem from Costa Rica with a purchased SIM but it never worked properly. We ended up buying their modem/SIM for about $20. Chile has the most expensive internet packages BUT it's really good, especially compared to Argentina (see below).
Purchased SIM card and modem at the official Entel store on the pedestrian street in Arica near the border with Peru.

Argentina: Moviestar
As of Jan. 2015, there were no cell carriers in Argentina that would allow us to buy/use a SIM card with a USB modem. We found someone to sell us a SIM card that we tried to use with our unlocked Claro modem from Costa Rica - it worked occasionally but not very well. Compared to it's neighboring countries, Argentina has very poor mobile internet. If you have an unlocked cellphone, you will have better luck than we did, and should choose the carrier based on the regions you will be visiting (just ask around).

Another good cross-reference for per country information can be found at: LifeRemotely.com

Mexico Telcel specific: homeonthehighway.com

Special Note About Unlocking A USA Phone
The cell carriers in the USA make it very difficult to unlock certain phones. The good news, is that many phones can easily be unlocked. Do some research to see if your particular phone can be unlocked and will work in other countries. One way, is to pay someone online for the unlock code. Another way is to have your carrier unlock your phone.

I was able to get Verizon to unlock my dad's old Motorola Droid smartphone when I told them that it was going to be used in Mexico with a different SIM. After it was unlocked, I went to the ATT store in the USA to get a prepaid SIM card so that we could also use the phone in the USA. When it didn't work, I did a little research and found that Verizon had the phone manufactured to block certain radio frequencies in the USA. The phone wouldn't work with other USA carriers unless you went through a laborious and involved process of rebuilding the phone's software and firmware. This process is a pain and I haven't taken the time to do it.

On a slightly related note, if you are leaving the country and cancelling your cell service, there may be a way for you to end your contract free and clear (look in the fine print). Ages ago, ATT refunded our early-termination fee when we proved to them that we moved out of the country.