Provided you have enabled plugins in Site Admin > Options all Supporters will have access to the plugin tab.
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 most of these don’t work on WPMU. Your installation comes with plugins that have been tested and customized to work on WPMU.
Here is a summary of some of the plugins they have available to use.
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
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 (http://akismet.com/download/ ). 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. Greet Box
Show 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
6. 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.
7. ThinkFree Viewer
The plugin allows users to view office(.doc, .xls and .ppt) documents on browsers
8. Threaded Comments
Enables threaded comments for your blog – simply activate, configure and enjoy as the comments perfectly nest.