I would recommend TeamCity - free for up to three agents, 20 projects and 20 users, runs a variety of builders (NAnt included) and can parse NUnit results (Hudson can do all this too I believe, however I have no used it, so I can't speak from experience).
Having worked with TFS, TeamCity, Bamboo and CC.NET, I can say that TC was the easiest to get up and running, the simplest to deploy multiple remote agents, get insight into builds, and integrated seamlessly with jabber, email, visual studio, windows task tray etc. Just felt good.