Normally, platform selection should be done after the application software has been selected. In this instance, the "application" was to provide a test bed for web applications of various kinds - possibly involving running different web servers and different database servers at different times. Since those applications weren't fixed, a platform allowing access to a broad range of those seemed to be called for.
The choices here are either a commercial Unix variant running on the vendors hardware, a commercial Unix variant running on IBM-PC class hardware, a free Unix variant, Linux, or Windows NT.
For small systems, the cost effectiveness of Unix vendors hardware was well below that of Unix variants on Intel. IBM-PC systems may have hardware stability issues that are better dealt with by Unix vendors hardware, but for my purposes - running a lightly-used web server sitting on site at my office - those weren't an issue. So the hardware platform is going to be an IBM-PC class system of some kind.
The commercial Unix variants offered no advantages except commercial support. In exchange, source was either more expensive, or unavailable. My experience has been that the support for popular software systems that have publicly available source is good. In contrast, working on a Unix system without source is not nearly as pleasant as working on one with source. For these reasons, the commercial Unix variants were dismissed from consideration.
At this point, the choices are a free Unix system - which means one of FreeBSD, OpenBSD or NetBSD - Linux, or Windows NT. Walnut Creek CD-ROM provides a comparison of FreeBSD, Linux and Windows NT. This agrees with my experience using those systems - Windows NT is both unstable and slow, and Linux is not quite as fast as a server platform as the BSD variants. This would leave nothing but the BSD variants, except for the possible requirement to run commercial applications. Commercial applications aren't as available for the BSD variants as for Linux. Both OpenBSD and FreeBSD provide Linux emulation, and the commercial applications of interest run on those. As an added bonus, since everything else will be built from source (or available in source form), converting from one of those two choices to Linux in the future should be simple.
The choice then comes down to OpenBSD or FreeBSD. FreeBSD is targeted at the server market - The power to serve being a common phrase on their server. OpenBSD seems to focus more on cross-platform functionality - which is not a small thing, but not of concern here - than issues that are important for being a server, which gives a slight edge to FreeBSD. When it comes to installing the applications software, both provide a ports collection, which makes this much easier. The OpenBSD ports collection was based on the FreeBSD one, which means FreeBSD came up with it first - another plus for FreeBSD.
With all those things considered - the choice was FreeBSD. The web server is currently running an a FreeBSD 3.1-RELEASE system; the database server on a FreeBSD 3.2-STABLE snapshot taken in mid-june.
Having selected the platform, it's time to select the software.