Plugins

In a multisite configuration, add a dropdown menu in administration to let you switch between sites admin.

WooCommerce Multisite module to synchronize product data using the SKU (stock, price...)

For WordPress MultiSite installs, allows accurate and private system crontab requests instead of classic WP-Cron.

Adds the ability to have different themes available to different NETWORKS in a multisite atmoshpere.

Ticket support system for multisite network.

This plugin adds a TOS (Terms of Service) field on the multisite signup form. (Such as wordpress.com TOS field)

Provides a system for registration requests and their processing in multisite. Two-level moderation.

WP3.1 multisite "mu-plugin" that allows you to add a user to all sites or no sites. Just drop in mu-plugins.

Automatically add users to peer blogs in a multisite network.

Simply add widgets in order to link another blog in a multisite instance