CruiseControl.net is a server that is used to run builds on software projects.

It works together with source control, NAnt, NUnit and other tools to build projects.

Here are some links to CruiseControl.net sites:

CCNet Main Site:
http://confluence.public.thoughtworks.org/display/CCNET/Welcome+to+CruiseControl.NET

How to set up a CCNet project:
http://joefield.mysite4now.com/blog/articles/146.aspx