The following are plugins written specifically for WPMU.

WP-LIVE-CHAT

Latest pictures lightbox

List recent posts (excluding pictures)

Login As...

QuckShop 1.2.4

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

Simple Dashboard Feeds

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

MU Admin Theme

Pick a Site

FireStats - web statistics for WPMU

WPMU Admin Dashboard

Global Theme Options

WPMU User Comment Tracking and Display

WPMU-login-captcha

OpenId plugin

Widgetbox Widget

More Privacy Options

WPMU Avatar Pack Release

Userthemes 1.1

Formattable RSS feed Recent Posts

Signup Security Question

Site-Wide-Categories

WPMU Sitewide Feed

Limit Country Signups

KB Notify Admin

My Comments and Trackbacks

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

The following are plugins written originally for WP Single but have been modified to work with WPMU.

MU - Advanced Admin Menus

Simple:Press Forums

Audio player for WPMU

lighter admin menus for WPMU

WPhone MU Edition

Valid SyntaxHiglighter for WPMU

WMPU Ldap+CAS Authentication Plug-in

Search Pages

Farms comment spam pack

WP-phpMyAdmin for WPMU

Embedded Video with Link MU

Most recent posts - updated

WP-PostRatings MU

Persistent Styles for WPMU

Sociable Plugin for WPMU

Swift-SMTP Mailer

WPMU Comment Timeout

Share This WPMU Edition

WPMU Livejournal Crossposter

Flickr Feeder

WP e-Commerce

Enhanced Links

Sitemap Generator Plugin for Wordpress(mu)

RS-Discuss

LDAP Authentication Plug-in

Auto Links Discovery Plugin

MU Admin Bar