If you are trying to keep your applications open source and free I would highly recommend using Spiceworks. It is a full fledged administrators tool but it also has a helpdesk function. You can customize it as you like and make it do exactly what you want.
You can check it out here.
This is from Spiceworks site about their help desk features:
ooking for simple but powerful help desk software? Something that lets you manage user tickets, organize your work, and assign tasks to your helpdesk team?
* Receive tickets via web or email
* Assign tickets across your IT team
* Create tickets while browsing network assets
* Track tickets by user, asset, due date & more
Spiceworks is completely free…and you can get started in less than 5 minutes. Download free help desk software now.