There is some great fan fiction out there....and some not so great...
For novel length fanfiction go to
Schnoogle@FanFiction Alley a great gateway to some really good fiction.
Trouble In Paradise/Paradise Lost by AngieJ is great...this is a classic and fannon....
The Paradigm Of Uncertainty/The Show that Never Ends (I think there may be more) by Lori is great (again another classic)
Slytherin Rising Series by JL Matthews is great, original/fannon tell the cannon HP stories from a slytherin original character's perspective, here is a bit of a twist, Ginny (Ginerva
) is in Slytherin. JL stopped writing for a bit, but is back on the horse and is in Year 4. I have stopped reading the series cause I want to read all 7 years in big swoop, but that could take years. So I call it pacing now.
Cassandra Claire had a great HP story out there, but she took it down off the internet after her original fiction was published.
These are all "classic" HP FF, so for new stuff you would have to look around FanFic alley....I can not recommend new writers.