I find myself repeatedly recommending the book
Feeling Good, by David Burns, because it is so helpful in fighting depression. It works by teaching you exercises (the pen and paper kind!) that help eliminate the thoughts that are at the root of depression. I find that whenever I do an exercise, my mood jumps a ton!
Anyway, I recently came across the site
The Mood Gym, which is a free site that uses some of the same methods.
I haven't really used it myself and cannot vouch for it, but I thought some members might be interested.