General:
* Humorous
* Amusing
* Hilarious
* Comical
* Entertaining
* Witty
* Clever
* Laughable
* Side-splitting
* Hilarous
* Rib-tickling
Informal:
* Crack-up
* Giggle-inducing
* LOL-worthy
* Snort-worthy
More specific:
* If it's cleverly written: witty, clever, satirical
* If it's physical humor: slapstick, farcical
* If it's sarcastic: ironic, dry, sardonic
* If it's absurd: outlandish, bizarre, ludicrous
Formal:
* Jocular
* Droll
The best word to use will depend on the specific situation and what kind of humor you're trying to describe.