I ran across this a few days ago and have been eagerly reading it, so figured I'd recommend it here too:

Harry Potter and the Methods of Rationality

It's one of the best, and funniest, pieces of fanfic I have ever run across. Brilliantly done and the author actually understands how science works - less common than you'd think even in science fiction... No TG, but still well worth your time.

Enjoy,