Google has always had this great sense of humor especially when it comes to april fools day (source of the moon.google.com) and such.
The only other one I personally know about is a bit geek but interesting. Type in "the answer to life, the universe, and everything" into Google. It's a Douglas Adams reference and pretty funny.
EDIT: Oh yeah, I forgot, Google also has a huge selection of languages to search in, including pig latin, swedish chef, klingon, hacker, and Elmer Fudd.
