My startup is currently developing a new ticket tracking system named Tracker, and it might be exactly what you're looking for. Tracker's focus is helping you do email support for external customers, but it also works well for internal company support.

Tracker is lightweight, fast, and has exceptional search and email integration. The web interface is quite quick and has some nice features, but you can also perform most functions (replying to customers, creating tickets, commenting) straight from your mail client.

We're currently in private beta, but if you're interested you can email me at dave AT madwombat DOT com, and I'll set you up with an account.