I was all prepared to impress you all with my knowledge of the two most humorous fanfics ever created: Half Life; Full Life Consequences, and My Immortal...and was promptly ninja'd by the OP.
OT: I don't know, the vast majority of fanfics are absolutely abhorrent, but I will say this. On occasion, you will find something fascinating. You will find, amongst a sea of fictions that are, arguable, absolutely horrific in their own right, a fanfiction so nauseatingly bad that it comes full circle to the point where it is actually perceived as amazing. These are, and forever will be, the only reason to even bother yourself with fanfictions, in my own opinion. For examples, see above.
Yes, I suppose there are some good ones out there, but they are so far and few between, it's really just not worth it for me.