CruiseControl.NET - it can be a bit of a pain to set up (as can most CI systems), but it is worth perservering with. I currently have it set up to run unit tests on completion of builds, and to produce Wix installers on-demand. As Dan said, it looks a bit dated, but that doesn't matter, as it provides you with plenty of information that's easy to get at and easy to read.
One thing - make sure all your developers have CC Tray installed, running and pointing to their builds. It's a great feeling to get "Another successful build" in your notification tray.
