I second this one but would recommend learning Javascript by using Greasemonkey and the introductory guide "Dive Into Greasemonkey" which was discussed here recently http://news.ycombinator.com/item?id=897706.

Greasemonkey lets you make changes to websites by writing small programs and it is fairly easy to find problems around the web to fix.

Example problem to solve: Hide posts on Hacker News from blogs that you already read elsewhere or that you don't like.