The Koha plugin system is a new long awaited feature that is likely to be in the 3.12 release of Koha! The plugin system gives Koha users the ability to add tools and reports to Koha that would be unlikely to become part of the integral community version of Koha! For example, a great use case for a report plugin would be the need a report that is too complicated to do in pure SQL. A good example of a tool plugin could be a catalog card printer. I’m sure once the plugin system has part of Koha that we will find many uses for it that I have not yetforeseen!
[update 4/23/2013] Check out the new Koha Plugins Database for plugins for your Koha installation. [/update]