If you want your projects to communicate over a distance, you have a few choices such as Bluetooth and WiFi. But if you want your project to communicate over a large distance, you're pretty much limited to using mobile data. Arduino have already developed a shield you can pop your SIM card into, but this is quite a big and expensive board.
Enter the Spark Electron. Made by the same team that brought us the Spark Core, the Spark Electron is a tiny Arduino based board, that has a built in SIM card slot. You can send little packets of data via 3G mobile internet, and although it's not officially supported, the Spark Electron should be able to make and receive calls and texts. It even comes bundled with a SIM and a bargain data plan, designed specifically for the Electron.
Featuring an ARM Cortex M3 processor, the Electron works similar to any Arduino board, offering 28 GPIO pins and serial communication over the TX/RX pins. The tiny board is 2" x 0.8", making it perfect for little set-and-forget projects, that work all over Europe, the USA and Canada. the electron has a choice of 2G or 3G chips, so you can save a bit of money if you only need 2G connectivity.
The included data plan, for $2.99 a month, allows you to send 1MB of data. This doesn't sound a lot, but if your messages are short, you can get up to 20,000 a month! The SIM isn't limited to the US, and can be cancelled any time. If you find you use more than 1MB of data, you can get an extra MB for $0.99, or you can stick your own SIM card in there and use that instead.
Spark have also developed a lot of tools to use with their Core and Electron boards to make it easy to develop using their boards. the code is based on Wiring, the same as the Arduino, and has a REST API for getting data in JSON format. You can also use their Web IDE to program the board, and use Tinker to tinker with your project via a smartphone. This makes the Spark Electron pretty easy for beginners to jump straight into.
The Electron is a great little project, allowing you to develop all sort of remote projects, from weather stations and bird feeders, to home automation and stolen bicycle trackers. The Electron is only $39 for the 2G model, and $59 for the 3G. Spark have already smashed their goal ten times over, so you can go grab one by pledging to their Kickstarter campaign safe in the knowledge you'll get one delivered to your door in October.