I've claimed Supernatural, and as I have a number of previously written fics, I thought I'd save us some time and space by linking them all in one post.
Supernatural/Castle
These have turned into the start of series.
It was a dark and stormy night (no, really!): 001 - Beginnings
That was the last time Castle ever wrote about vampires. The real thing just didn't sell like Anne Rice. (PG, 434 words)
Hell's a'Blazing: 060 - Drink
When the FBI comes to take a case out of Beckett's hands, a spark of recognition on Castle's part puts the "agents" in a tight spot. (PG, 1073 words)
Consulting Mr. Castle, Come in Mr. Castle: 073 - Light
Sam and Dean borrow Detective Beckett's consultant for a little bit of questioning. (PG-13, 1082 words)
Supernatural/Doctor Who
The Angels Have the Phone Box: 021 - Friends
It's his TARDIS, not a free-for-all time machine. But since they ask nicely, he lets them borrow it. (PG, 585 words)
And a three-part series:
A Fourth Part of the Earth: 062 - Spring
The Doctor and Martha come across an outbreak of swine flu, at the wrong time, which confuses the Doctor. So does the stranger who comes out of nowhere telling them to butt out - it's not their fight. (PG, 1188 words)
To Talk of Many Things: 010 - Years
Castiel wakes up in a very strange place. A... time-traveling spaceship. He's a little curious. (PG, 1118 words)
Tap Your Heels Together Three Times: 024 - Family
The Doctor and Martha get Castiel back to Singer Salvage yard, and find that 2010 is full of surprises. (PG, 1584 words)
Supernatural/Firefly
Changing Channels--Firefly: 022 - Enemies
During "Changing Channels", Dean and Sam get caught in a rerun of a canceled FOX sci-fi show. (PG, 699 words)
Supernatural/Greek Mythology
Ancient History: 013 - Yellow
Dean and Sam think they've found another one of the Trickster's old pranks, but a bit of archangel interrogation turns up a different suspect. (PG, 848 words)
Supernatural/Psych
Joke's on You: 035 - Sixth Sense
A fake trickster/hiding archangel pulls one of his pranks on an unsuspecting "fake" psychic. (PG, 902 words)
Supernatural/Star Trek
Changing Channels--Star Trek XI: 015 - Blue
During "Changing Channels", Dean and Sam get caught in a TV premiere of the newest Star Trek film. (G, 469 words)