If you were at all amused, or shocked, or maybe even slightly traumatized by my weeklong adventure at 100Mhz, you might keep an eye on the Lightweight Linux blog, which is documenting a similar experiment: NetBSD on a K6 machine.
I have no experience with any of the *BSDs, although it is on my to-do list. If I find the time to uproot myself from Linux and try out Net- or Free- or Open-, I’ll probably take another look at that site, as an example.
The key words there being “if I find the time.” Lately, that seems to be the problem. … 
NetBSD is awesome on just about any machine. And it runs on a whole lot of machines