We use MSBuild, because we started with Visual Studio 2005 (now Visual Studio 2008), and MSBuild was already "built in" to the SDK - there is less maintenance on the build server. It's a NAnt clone, really - both tools are infinitely flexible in that they let you create custom build tasks in code, and both have a decent set of community build tasks already created.
- MSBuild Community Tasks
- NAntContrib
