Rake script
Fluent NHibernate is distributed with a Ruby Rake script, which can be used to build without having to open Visual Studio; more importantly, it also provides a simple mechanism for switching versions of NHibernate (see the supported NHiberate versions for more details on our versioning policy).
Before you can run the rake script, you need to make sure you have all the necessary gems installed; the easiest way to do that is to run the InstallGems.bat distributed with Fluent NHibernate.
Once you've got all the gems installed, you can either run Build.bat which just runs rake, or you can run rake directly: rake. Either of those will use MSBuild to build the solution, and then will run the unit tests before copying the built files to the build directory.
When the build is complete, you can reference the Fluent NHibernate binaries from the build directory.