Taken from: The WordPress MU Manual
Plugins extend and expand the functionality of WordPress.
There are two types of plugins; regular plugins and mu-plugins. These are located in separate folders on your site and work differently.
Regular plugins are located in the “plugins” folder; they can be activated and deactivated on a per-blog level using the admin plugin page. Mu-plugins, on the other hand, are the plugins located in the “mu-plugins” folder; these are activated by default affecting all blogs on the WPMU installation.
While there are over 4,000 regular plugins many of these don’t work on WPMU.
Those known to work on WPMU include:
1. AddThis Social Bookmarking
The AddThis Social Bookmarking plugin allows readers to bookmark posts from blogs easily with over 45 popular services, and to share it with friends via email
2. Akismet
Akismet checks your comments against the Akismet web service to see if they look like spam or not and lets you review the spam it catches under your blog’s “Comments” admin screen. To enable Akismet you need to follow the instructions to obtain a WordPress.com API key.
3. Audio and Video Comments
Lets readers to leave audio and video comments.
4. Contact Form
Plugin that allows user to set up a contact form that can be embedded into a page by Douglas Karr.
5. Google Analytics
All Campus blogs include stats tracking which collects information about the number of visitors to the blog, where they are coming from, the most popular content and the search engine terms used by people. This can be accessed through the Stats Tab to the right of Comments.
For more detailed statistics users can activate the Google Analytics plugin.
6. Greet Box
Greet Box works by showing a different message to your visitor depending on which site they are coming from. For example, you can ask Digg visitors to Digg your post, Google visitors to subscribe to your RSS feed
7. Subscribe To Comments
The Subscribe To Comments is a plugin that allows commenters on your blog to check a box before commenting and get e-mail notification of further comments.
8. ThinkFree Viewer
The ThinkFree Viewer plugin allows users to view office (.doc, .xls and .ppt) documents on browsers
9. Threaded Comments
Enables threaded comments for your blog – simply activate, configure and enjoy as the comments perfectly nest.
Any other regular plugins you want to try should be thoroughly tested by uploading to your WPMU site, and enabling it just for a test account. Activate the plugin on the test blog taking notes of any errors which occur. Immediately remove any plugins with bugs and only use them site-wide once you have fixed all their issues
