Most regular WordPress Plugins do work with MU. Others can be made to work with a few bits of code.

• WPMU Tutorial - A tutorial on writing options screens for plugins so that the plugin works in both WordPress and WordPress MU.
• WPMU Tutorial - Part 2 of the above tutorial on writing options screens showing how to implement an option array in an option screen. Also works in both WordPress and WordPress MU.

This is just a sampling of some WordPress MU Plugins. Please check the WordPress Plugin Directory for more.