More than you ever wanted to know here...

http://en.wikipedia.org/wiki/Ruger_10/22
http://tinyurl.com/34hjfh

It's a rifle, but for $261 list (new and used for much, much less elsewhere... see tinyurl above), it is one of the most versatile, customizable, and fun .22 rifles you can find anywhere. Highly recommended for anyone on a budget who wants a quality kick in the pants.

The links at the bottom of the wikipage are a good start for more info.