TeamCity is a very good CI server. (and the "Professional" edition is free)
I've been using it for over a year for building .NET projects, and I must say it's way better than CC.NET IMHO.
Strong points are:
- Very easy to configure (web based GUI)
- Distributed system (you can have several build agents on multiple machines to distribute the build process)
- Built-in support for many source control systems
- ... check the website. The product is awesome ...
