The Raspberry Pi is a full-fledged computer for $35. You can use your existing keyboard, mouse, and screen with it, and so much more.
If you ever wanted to learn more about computer hardware, electronics, or computer programming, it's a great place to start. There are all sorts of simple projects that you can learn to make, and you can do some amazing things with it. Check out some of the projects people have made in the Lessons and Community Projects tabs.
Also, if you ever get stuck on a project, there's a vibrant community. Check out some of the Frequently Asked Questions above. You may find the answer easier than you think!