+1 for Hudson.
Hudson is a very active project, has a wide community of users and an active users mailing list, is really easy to start with, is easy to use, has been used on huge, very huge, projects (JBoss, JAX-WS, etc) and thus has proven records of success, offers very nice advanced features (e.g. build matrix, build clustering, etc), is open source, has a lot of plugins...
And if support is really an important thing, you can get commercial support from Sun. But FWIW, I never faced any blocking problem with Hudson.
Update: As you may know, Kohsuke Kawaguchi (the creator of Hudson) has left Sun/Oracle and started his own company to take Hudson to the next stage. In other words, this is not a threat for Hudson. And if you are looking for support, you can get a certified version of Hudson CI Server as part of a subscription plan (this certified version bundles a high quality release of Hudson with a predefined set of plugins plus some commercial one).
Update: To illustrate the size of their respective user base, here is a comparison of job trends for several CI tools on Indeed (live query):
This is of course not a technical indicator.
