Praqma uses Jenkins every day and in many ways. We have written a handful of plugins for Jenkins and we have also contributed with extensions to the Jenkins core.
If you want to see a live Jenkins server, you are welcome to take a tour on our own setup, where you'll have read access to all of our Open Soruce projects:
Or you can give it a test Drive simply by hitting this button:
It will Launch Jenkins through Java Web Start for a test drive. Once it launches, visit http://localhost:8080/ to get to the dashboard. Any configuration that you do with this Jenkins will be stored in ~/.jenkins, so your data will survive through Jenkins process restart.