Trouble Ticket Systems?

Zen Master Nate asks: "I work for a startup ISP, and part of my job is support. A trouble ticket system would be quite helpful, but all the ones I have found are overpriced and way too complex for our needs. My question is: What trouble ticketing systems do you use, commercial, freeware/open source, or in-house? Also, if I were to write one, what features are needed, which ones are useful, and which are unnecessary? "