Basically what the title says! Humor can range from standard humor (the characters are in a ludicrous situation and/or are cracking a lot of jokes), parodious (whatever...) humor (stories which basically use the characters to mock the show/movie/other characters), or basically anything funny!
XF/Monty Python. I've read this at least five times, and I still crack up at it. Hilarious, and we see what we all want to see...a horrible death by stoning for Diana Fowley. Actually, several Fowley death scenes. And there was much rejoicing.
155 Words or The Shortest X-Files/Buffy the Vampire Slayer Crossover
Basically what the title says! Fun for the whole family. Short and sweet, for Empowered Women! on the go (joke from the story, sorry).
Can't tell you what this is a crossover with, because it'd spoil the surprise. I love this one.
Delete This File!
Primarily an XF/Star Trek (TOS and TNG) crossover, but includes a few cameos as well. NC-17 for not-too-explicit sexual details, quite funny. Personally, I'd only give it an R, but it's the author's decision.
XF/Blue's Clues crossover. Steve is wrongfully accused of murder, and only Blue knows the real killer. You can guess what happens.
The Pooh Files
XF/Winnie-the-Pooh (the books, not the Disney movie) crossover. One of my favorites.
Greetings From Earth
XF/Third Rock from the Sun crossover. Another one of my faves.
A Teletubby X-File
XF/Teletubbies crossover. Quite entertaining.
A Very Brady X-File
XF/The Brady Bunch crossover. One of the first I ever read, so I guess it's partially responsible for warping me into the crossover-obsessed freak I am today. But in a good way.
Very Terrible Crossovers
Various crossovers. This one had me in tears of laughter.
XF/Who Wants to Be a Millionaire crossover.