For our engine, Neomo, we’re using Linux (mostly Debian-based) with kernels 2.4.x as well as the new 2.6.x versions. I once tested FreeBSD, but did not find any advantages that justified any further engagement.
There once was a german engine called Acoon, that was running on a single box using Windows 2000 and the MS SQL Server.
What system is your engine running on?