What is a good open source help desk system?

Hello,
I do tech support for a small buisness in CA. I am thinking of using a help desk system that can be accessed by other employees. I would like to manage technical support issues, contact system, notes, maybe even a calander.

What do you think is a good system? I want open source first to test the waters and make sure the system will be used. If so I will then switch to a full help desk system.

Thanks.
I guess I sould mention, I am looking for help desk software not a different OS.