Nothing Like Billy, by Embroiderama. Het, Ellen Harvelle/Fox Mulder. (ca. 2,622 words)
Supernatural/The X-Files
There isn't much plot beyond the sex in this crossover, but there was enough characterization and a plausible reason for their meeting to not have the story feel arbitrary.
Leapin Lizards, by Gwendolyn Grace. Gen, Sam Winchester, Dean Winchester, Sam Beckett, Al Calavicci. (ca. 18,560 words)
Quantum Leap/Supernatural
Sam Beckett leaps into John Winchester when Dean and Sam were still children. There is a good case story, and Beckett has to deal with finding out that the supernatural is real, and handle two young Winchesters.
With a Bang, by Mink. Gen, Alec, Dean Winchester, Sam Winchester. (ca. 42,000 words)
Dark Angel/Supernatural
This is a good action-adventure, and I liked how the shows' mythologies were meshed, even though I'm not that into the whole breeding cult arc of DA season two. Also Alec, Sam and Dean together are just fun to watch.
Curiosities (ca. 4,045 words) and
Stutter (ca. 1,500 words), by Penknife. Slash, Erik Lehnsherr/Charles Xavier, Tom Riddle, Lily Potter, Sirius Black.
Harry Potter/X-Men
Recently Penknife offered to write timestamp fic, and I noticed that I never recced the original crossover for which I requested a timestamp. Anyway, in the first Erik Lehnsherr encounters Tom Riddle in 1950, the timestamp is set during the first Voldemort war/the Marauders era, and both are interesting and well worth reading.
Brothers in Arms, by SylvanWitch. Gen, Dean Winchester, Derek Reese. (ca. 2,440 words)
Supernatural/Terminator: The Sarah Connor Chronicles
Dean runs into Derek while hunting, and somehow these two shows just go together like chocolate and hazelnuts.
What Kind of Day Has It Been?, by Victoria P. Gen, Dean Winchester, Sam Winchester, Kermit and the Muppet Show ensemble. (ca. 3,300 words)
Muppet Show/Supernatural
I love Muppet crossovers (felt rules! *g*), and this one made me laugh a lot.