Some machines I've played with FreeBSD on:
You might find the information here interesting (although it is primarily to remind myself of what I did before senility sets in about 40 years from now). I know, this reeks of geek-showing-off-his-toys, but be honest - that's why you're here.
Toshiba Portege 7020CT (333MHz PII, 192MB RAM, 20GB HD)
Dell Dimension XPST450 (450Mhz PIII, 384MB RAM, 60GB HD)
Dell Dimension 8100/Limited (1.3GHz P4, 128MB RAM, 40GB HD)
IBM ThinkPad 600 (233MHz Pentium, 160MB RAM, 6GB HD)
Compaq Deskpro 4100 (66MHz i486DX4, 36MB RAM, 408MB HD)
Compaq Prolinea 4/66 (66Mhz i486DX2, 20MB RAM, 327MB HD)
Why FreeBSD?
I spent 5 minutes figuring out that FreeBSD has one distro, the installer is clean and functional, and the FreeBSD 'packages' and 'ports' system provides a versatile and simple mechanism for installing a huge range of useful software. You can do big and friendly installations on modern machines, or rejuvinate old 486-class machines as network servers and gateways.
Xfree86-4.1.0 and KDE/GNOME. What else needs to be said? I fell into the KDE desktop camp, and use Kdevelop2.0 as my primary C code hacking environment (it isn't quite as robust and powerful as MS Visual C++ 6.0 under Win32, but you can't beat it for price). All this can be easily installed from pre-compiled FreeBSD packages.
True, it isn't Linux. Not a bad thing if you're running a server. Not entirely a disaster if you're building a desktop machine - Linux binaries execute alongside native FreeBSD binaries under FreeBSD. So I get the clean install of FreeBSD, and run Linux versions of Netscape, StarOffice, QuakeIII servers, etc.
So hey, what's stopping you from doing this too :-)
updated 12/31/01