Sooooo many and they make the game worlds more fun, which means it turns an already amazing game into an even more amazing game.
My favorites are added Force powers, character changes -- like one where you can turn Mission into Darth Talon (well only in appearance, she still talks like Mission unfortunately, but she's a dark Jedi and looks awesome), and there's a nice mod that lets you be a Jedi from the beginning. Also stuff like new weapons, new lightsaber colors, new robes and armor and clothes and oh man so much stuff.
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~Halo Now you learn the mistake of challenging me.
http://img408.imageshack.us/img408/2734/hieis.jpg