I've been recommending to clients that they consider Bamboo. The reason being that (ok, from reading the spec sheets!) it has a very similar feature set to TeamCity. However the main benefit is very tight integration with JIRA which is quite popular as a feature/bug tracking system. The complete suite being JIRA, Greenhopper, Bamboo and Eclipse. Quite a few clients also have HP Quality centre and there are plugins which joins that to JIRA as well. I also like the fact that JIRA, Bamboo and GreenHopper all come from Atlassian.
