I’ve been playing with these tiny little computers for over a year and very pleased & amazed with the capabilities. The Raspberry Pi is a credit-card sized computer selling for about $35. You can run it without a case or buy one for $10-$20. All you need is a $10 SD memory card and your set.
This tiny computer sports USB ports, Ethernet networking, HDMI display, sound, 512 MB memory (there is also a 256 MB flavour which is about $10 cheaper), and a general purposes input/output (GPIO) header block if you want to connect/control other things.
There are a variety of linux distibutions you can download and image to an SD card resulting in an instant server with a large number of possibilities for uses. (Google “Raspberry Pi” and “LEGO” for fun.)
If you are a hobbyist and want to have some fun, get yourself a Raspberry Pi and see what your creative juices can conjure up!