As a company that maintains a large in-house software development team (for Staff.com and Time Doctor), we have always been interested in bug tracking software. Two years ago, we started a public spreadsheet of bug tracking tools where anyone check out and compare the best software for them. The spreadsheet includes popular paid options like Jira and Microsoft Team Foundation and also lots of great open source alternatives.
Jira has an interesting sales model where they offer the first 10 licenses at a minimal cost, and then charge for add-ons or for larger teams. I actually visited their office in Sydney, Australia recently and they are a very successful company with around $100 million in revenue!
I hope you will find this list useful. If you have anything to add to the list, please let us know.
You can view this spreadsheet on Google Docs: