I've started reading a crossover series that I quite enough, so I thought I'd begin a topic for specifically that subject. I'm an avid fan of fanfiction crossovers for about a dozen verses so this list might become, shall we say, diverse. But, I'll start with my newest favorite.
The Demented!Verse stories beginning with Damned Demented Demons
by hells_half_acre over at Live Journal
Universes: Supernatural and Harry Potter
Link:
http://hells-half-acre.livejournal.com/190948.htmlSummary: In the first story, "Harry saves the Winchesters from a Dementor attack, but the Winchesters aren't the only ones that need protecting." Basically, it's set after Harry and the gang have settled down and started their production line of look alikes *cough* kids. Anyhow, it's a great series. Very funny gen fic (no pairing that aren't in the HP epilogue as far as I can tell). The second story in the series is especially fun--it's a short piece consisting entirely of letters between Bobby and Hermione.
Warnings: Big spoilers for Supernatural. Drops the f-bomb on occasion. Same amount of violence as HP or SPN.