It's a mixed bag, but some of the authors I've been finding decent at ff.net are posting over at castle_fic. There's also some stuff that's not showing up at the cesspool that was amusing and pretty well-written. Check out "Mad Doctors and B Movie Nazis" when you get there... so long as you've seen "The Third Man" first. =)

ETA: I think I just read what may be the funniest Castle fan fic ever. The latest installment, called "Nick & Nora, Bond & Vesper," can be found here. The links to the earlier stories are in the Author's Notes, and I highly recommend those. Characterization was pretty consistent, and I love the merry little war the author stirs up between Ryan, Esposito, and Castle. Some portions -- and the accompanying mental images -- had me laughing out loud.