The Network Plugin Auditor lets you track which plugins were used on which sites. It adds columns to your network admin to show which sites are using each plugin, saving you the trouble of having to visit every dashboard to find out.
Would be great to test this little baby out to help us during our upgrades: http://wordpress.org/extend/plugins/network-plugin-auditor/
Multisite Plugin Stats is similar: This is a simple plugin that shows plugin activations across all your sites. It is particularly useful for checking to see which plugins are not being used at all so that you can deactivate those as needed. You can also find out which are the most popularly used plugins on your network.
Download Multisite Plugin Stats here: http://wordpress.org/extend/plugins/multisite-plugin-stats/