MarketPress

• The "MarketPress" plugin lets you turn your WordPress MU site into a fully featured e-commerce website and includes a shopping cart to make online transactions simple.
  
  

Membership

• The "Membership" plugin gives the tools to start a membership site on your WordPress MU installation. It is also supported on the standard WordPress install and compatible with the "BuddyPress" plugin. You can offer downloads, give users special privileges to access member-only posts and pages, and accept payments online.
  
  

Pop Up

• The "Pop Up!" plugin lets you display a pop-up dialog box over the content of the WordPress site. You can customize it to show throughout all pages and posts of the site, on individual blogs or on a single post. This type of plugin is helpful in collecting email addresses from visitors.
  
  

Easy Google +1

• The "Easy Google +1" plugin lets you easily add the Google +1 button to every page and post of your WordPress site. The +1 button is part of the Google+ social network and shares a link to the content with that Google+ user's circle of friends.
  
  

Advanced Theme Switcher

• The "Advanced Theme Switcher" plugin does exactly what its name says. You can lets WordPress MU users quickly change their blog's theme. You can make as many themes available to users as you would.
  
  

Select Language At Signup

• The "Select Language At Signup" plugin is useful for WordPress MU sites that cater to visitors from multiple countries, especially non-English speaking countries. The plugin gives a simple option during the user registration process that asks what the user's preferred language is.
  
  

More Plugins

• Find more WordPress MU compatible plugins by visiting wpmudev.org in your browser. Then hover over the "Plugins" tab on the site's top navigation bar and click the "All Multisite Plugins" option to see a comprehensive list.