Okay, people. Here's a lesson in evil laughter.
If you have a tyrant, s/he should laugh in such a manner as to perpetuate his/her status. The laugh should be full-bodied and such that it throws the head back, reinforcing their position of authority. It should go something like this:
MMUUHHOOHHAHAHAHAHAAA.
It begins deep and ends deep and should have limited hysteria. Notice, there's no exclamation point. Essentially, this is your Lawful evil mastermind.
If you are dealing with a villain with delusions of grandeur, you will have a laugh similar to the tyrant, but not as deep:
HHHEHEHHAHAHAHAHAHAAAAHHH!
This is your Lawful evil lacky wannabe or your Neutral Evil mastermind.
Now, for the villain that's all about power for himself, and into Chaos, this laugh is very effective:
HUUUHHAHAHAHAHAHAAAAAHHH!!
This laugh is full of hate and malice just barely contained. This is a laugh of a battle-raging Great Red Wyrm.
The mad scientist laugh is always fun. It comes in two varieties. The classic:
MMWAAHAHAHAHAHAHAHAAAAA!!!!!
And the newer
BBWAAHAHAHAHAHAHAHAAAAA!!!!!
These both work fine, although the latter one is for the villain on the up and coming.
Now, for purists interested in the completely evil, depraved, loony-toony laugh:
HHHHAAAAAAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAAAAAAAAAAAHHHHHHHHHHHHHHHHHH!!!!!!!!!!!!!!
Clearly, this is The Joker laugh. Quite honestly, if you want a truly insane laugh, one doesn't get any better than this.