I know that Wikipedia has been getting a lot of bad press lately, but I've personally never had too many problems with it. The Uncyclopedia is a fun parody, though. When you arrive, you are greeted with the message:
Welcome to Uncyclopedia, the content-free encyclopedia that anyone can edit.
... it seems to be rather well put together, so far.
Warning: some of the content may be offensive. I've not dug too deeply, I went straight for the IBM definition. The definition of Linux (or rather, DorkWare for Dweebs) is pretty amusing, too :-)
(I first picked up news of the Uncyclopedia over on James Governor's blog)
Technorati tags: wikipedia uncyclopedia ibm linux joke parody