Team City is by far and away the best CI server out there. Its killer feature for me is the tight integration with IDEs (IntelliJ, Eclipse and VisualStudio). It can show you, for example, when a file you're editing in the IDE becomes out of date, who changed it and what they changed. You can commit from the IDE to the CI server, run the comile and tests on the build grid, and then the CI server will commit if the build is successful. You can click on build reports in the CI web app and it will open the appropriate files in the IDE.
There are plugins available (I wrote one: http://team-piazza.googlecode.com), but not many.