Ooh, how have I never heard of Harry Potter and the Methods of Rationality? Anyhow, I downloaded it, can't wait to read it and be transported back, for a brief while, to the days of new Harry Potter books.
Don't have much to recommend, as all the fan fiction I've been exposed to has either been awful or x-rated (sometimes both!). However, I totally recommend reading the novel "Fangirl" by Rainbow Rowell, along with its fan fictionesque spinoff, "Carry On". Also, "Harry Potter & the 10 Years Later" is a highly entertaining video series.