Zephyr is a test management system -- a project management and collaboration system for the Quality Assurance crowd. I've blogged about several PM systems in the past, but this one seems specialized for QA and might fill a niche that the more general PM systems don't. Currently you can get a free download with three licenses.
Unlike most of the PM systems I've discussed, "Zephyr is a Windows-based server with rich interactive clients that run in any web browser." So you keep your copy on your own server, eliminating the security concerns that people have with hosted PM applications such as Basecamp and TeamWork Live. On the other hand, you need a Windows server. Since we don't have one here at the CWRL (we're using BSD), I have not tried the product. But if you do, and your team does QA, check it out and tell me what you think.