Seven Moments, by Barrowjane. Gen. (ca. 1,300 words)
Sandman/Star Trek: Reboot
There's just something about other characters meeting the Endless that I really enjoy, and this example of the genre is no exception.
Down The Rabbit Hole, by Bluflamingo. Slash, Colby/Lorne, Colby/Daniel. (ca. 10,000 words)
Numb3rs/Stargate: Atlantis/Stargate: SG-1
I like how this handles the relationships, the complications that come with dating outside of your secret alien conspiracy, and the way the SGC happens to tangle with the FBI here works for me too.
People Who Repair Quantums, or Five Planets The Enterprise Never Visited, by Ignipes. Gen. (ca. 2,800 words)
Star Trek: Reboot/Alien/Discworld/Doctor Who/Dune/Star Wars
I really enjoyed the crossovers for which I was familiar with the sources, also, while I don't know Discworld, the bit where Spock reacts to the flat disc and giant turtle and so on in space was awesome.
Bright White Light, by lilacsigil. Gen. (ca. 35,000 words)
Supernatural/X-Files
This is set during the X-Files episode "Fallen Angel", which I barely remembered, but the story still worked for me. We get the events of the episode but seen from the perspective (and worldview) of young Sam and Dean. I thought it was really cool how one side believes it's all aliens, and the other it's all supernatural and I was wondering what was really going on. And I liked Sam and Dean adventuring.
Exiles, by nwhepcat. Gen, Oz, Sam. WIP
The premise here is that Sam runs into Oz during Mystery Spot. I found this via Cofax and am really enjoying it so far. The parts are all quite short, but it is updated very often.
a fucking space opera (or Six Characters Who Didn't Board the USS Enterprise), by Oxoniensis. Gen. (3,169 words)
Star Trek: Reboot/Harry Potter/Merlin/Stargate Atlantis/Supernatural/Terminator: The Sarah Connor Chronicles
In the best tradition of wacky crossovers, this was simply a lot of fun.