Compared are Linux 2.4, 2.6, FreeBSD, NetBSD, and OpenBSD. Really well-performed benchmarks, with graphs.

http://bulk.fefe.de/scalability

Linux 2.6 was hands down the winner, which makes me feel good about Slackware (2.6 linux but actually stable) as a server. I'm sure Windows would have won if only it was benchmarked. One thing to keep in mind - from what I gathered, the machine tested was a single-processor, single-core machine...this means that SMP scalability was not tested, a HUGE consideration for more modern servers (what server now doesn't have multiple cores?) and may skew the modern-day results, especially between the two leads, FreeBSD and Linux 2.6.