Recently I decided to get a Raspberry Pi, which for those of you that haven't already heard of it, is a fully functional computer about the size of a credit card, the latest version is the Raspberry Pi 2 model B, despite the small size it has 4 USB 2.0 ports, ethernet, HDMI video up to 1920x1200, 3.5mm audio jack, micro SD card slot and a micro USB connector for power.
The specifications are fairly impressive as well featuring a quad core ARM Cortex-A7 clocked at 900MHz, a Broadcom VideCore IV clocked at 250MHz and 1GB of RAM which is shared with the GPU, easily enough grunt for full HD video playback and a good number of games and applications.
Raspberry PI 2 Model B The Pi doesn't come with any operating system built in, however officially Raspbian is the recommended OS which is a stripped down version of Debian Linux, there are a number of other alternatives such as Arch Linux, RISC OS and Puppy Linux with Windows 10 being planned for free.
Installing Raspbian is quite straightforward, you can download the archive from the official website and copy it on to a fat32 formatted microsd card, installing only takes about 5 minutes and when it's done you get to set some options such as whether to boot to the terminal or the desktop.
I wouldn't really call the Pi easy to use, particularly if you've never used Linux before, things like getting a wireless adapter to work can take quite a bit of effort if you're unlucky but its usefulness outweighs the learning curve, currently I'm using mine as a torrent box and media center which for the price works extremely well.
A number of people have compared it to the Arduino but frankly I don't see it as being that useful in that respect, a header is provided which allows you to interface with it using GPIO, I2C and other protocols but its IO performance is not all that great, certainly there are some good uses such as a wireless weather station but for general use most people will be better served with an Arduino or similar.
Overall I can highly recommend the Raspberry Pi, it has such a wide variety of uses that it's very much worth getting even just to play around with, the price has recently dropped to $25 so now is the perfect time to buy.
Also a small bit of advice, if you're buying a wireless adapter find out what chipset it uses, then Google it to see what the Linux support is like, getting a well supported adapter can save you many many hours of frustration.