I think it is fair to say CruiseControl.NET has become synonymous with Continuous Integration in the .NET space. In the recent DNR on CI* with Eric Sink and Martin Woodward it was the defacto choice for the discussion and with good reason. CCNet is a great tool.
I just wanted to give a quick shout-out to Draco.NET which is also a CI server for .NET and while not having the broad featureset of CCNet I recommend it for smaller shops getting their first Continuous/Automated build together. My two reasons are:
- Supports Visual Source Safe, which like it or not is the Gateway Drug for source code control. (To hear what I think of that go grab DNR #221 about 5min16sec in ;) It also supports PVCS, Subversion and Vault.
- It is so much easier to configure. You have no options for reporting or postbuild steps, or overriding the build ID or RSS add-ons for. CCNetConfig is essential for configuring CruiseControl.NET. Draco.NET offers a pit of success for the folks new to CI.
The most compelling reason for CCNet over Draco.NET is Everyone else is using CCNet 
On the topic of DNR show #238 - Eric talks about Vault, but doesn't mention that you don't need to purchase a license when used for a single developer. It is licensed by named accounts, so go get it and give it a go all you VSS folks!
*the best bit of the show is 11min45sec in... 
Listening To: Ani Difranco - Revelling and Reckoning