I just started using TestLink at work. It's a software test case management (TCM) tool, something every QA team needs, in my humble opinion. We didn't have anything in place, so I researched and chose TestLink over a few other free/low cost solutions. It's not HP Quality Center, but so far it is getting the job done. I've only been using it for a few weeks now (we're in the middle of a sprint), so I'll give an update later, and a more thorough review of the tool.