The following are plugins written specifically for WPMU.

Wordpress MU Plugin Filter

Moderate New Blogs

Sitewide Comment Tracking Revisited

WPMU for Dummies: Part Deux

Themes in backend sorted by tags

Sitewide Multi Widget

Email Confirmation

Sermon Browser plug-in

xmlrpc Addons

Limit Post Revisions

Limit Users Per Blog

WPMU Custom Meta Tags

Standard XML Sitemap Plugin

Menus

More Privacy Options

All text allowed

Featured Posts

WP-reCAPTCHA

Rollover Themes List

WP-LIVE-CHAT

Latest pictures lightbox

List recent posts (excluding pictures)

Login As...

QuckShop 1.2.4

Simple:Press Forums

Plugin to off-load theme files to Amazon-S3

Allow Embedded Videos

Free Music Widget

Add User Widget

Bug fix for mu-massmailer

Mawker

List All Plus Widget

Sitewide recent comments

Removes the option to edit permalinks

5min video plugin for WordPress-MU

AdSense-Deluxe2

One Search WPMU Plugin

WPMU Sitewide Feed Plugin - ITDamager

WPMU new blog settings plugin

MuTags

WPMuLoginMC

Sitewide Tag Solution

WPMU Random Blog Redirect

WPMU Random Captcha

GRavatar plugin

WPMU Invitation

Plugin Commander

eBlogs plug-in

Change Blog URL

Syntax Highlighter plugin for WordPress-MU

User Groups

BlogRush widget for WordPress-MU

Pick a Site

FireStats - web statistics for WPMU

Global Theme Options

WPMU User Comment Tracking and Display

WPMU-login-captcha

OpenId plugin

Widgetbox Widget

WPMU Avatar Pack Release

Userthemes Revisited

Formattable RSS feed Recent Posts

Signup Security Question

Site-Wide-Categories

WPMU Sitewide Feed

Limit Country Signups

KB Notify Admin

XML Sitemap (google compliant) for WPMU

GT Search All Blogs

WPMU Site Wide Tags

googleSitemaps for WPMU

WPMU-Signup-Captcha

Wysiwyg Print

Blog categories

Ada A Blog Recent Posts Widget

Alexa Domain Rank

Dispoweb-Google-Alexa M-Rank

SMTP Mail

Multi-Site Manager

WPMU Anti Splog

WPMU Latex Math

Most recent posts

Theme Stats

Z-Space Upload Quotas

Total Upload Space Remaining v2.0

Snap Preview MU Plugin

Search

AffiliateMU

Mass Mailer

WPMU Odeo Plugin

Anarchy Media Player

Orangecows-sitemap

WPMU Recent Posts

List All [widgets]

Invitation plugin (skeleton)

Friend-List

Feedback

List All Posts

Upload Space Plugin

Plugin Manager

User Themes

List All Themes

Fimii WPMU Stats

List All