No question like this is complete without a link to the big CI Feature Matrix(Web Archive) which lists just about every CI option out there.
But I think it is important to look ahead to the scope of what you want to include in your CI system. Is it going to be just builds or are you going to be bringing in other elements like static analysis, cross-project dependencies, deployments, functional tests, etc. To help with that planning I created this wallchart on the Elements of Enterprise CI (PDF; no registration required). Please don't let the "E-word" put you off; I just mean stuff beyond the basic fast feedback CI build. :)
It isn't tool specific but lists a variety of practices you might consider while you're in the planning/evaluation stages.