From Koha Wiki
Plugins for use with Koha
Kyle Hall did an introductory video on Plugins here: http://bywatersolutions.com/2013/01/23/koha-plugin-system-coming-soon/
Also the manual covers plugins in detail: 
If you are interested in developing a Koha plugin, a good reference is the Kitchen Sink plugin which implements every feature of the plugin system.
You can also use Travis CI to automatically build and release your kpz files for you, using this guide
- Koha EDS API - This plugin integrates EBSCO Discovery Service(EDS) in Koha.
- Donor Tracking - This is a plugin used to track donor contributions and contacts. More information at Donor Tracking wiki
- Counters - Display counters of things happening in the library (currently loans and searches) on screen in the library.
- Room Reservations Plugin - This plugin provides a room reservation solution on both intranet and OPAC interfaces.
- Koha Plugins Database from ByWater