Virtual PC is the primary contender. It's pretty cool in that it emulates WinTel hardware rather than the OS itself, so you can install whatever copy of Windows or Linux (more on that in a moment) you want. It's a little slow but it should be fine for PBEM.
The downside is that Connectix was bought out by Big Bad Billy Boy and now Micro$oft owns it. Linux is no longer supported, naturally... although supposedly it does still work.
Bochs is an open source alternative. Supposedly very slow, however. For PBEM, it shouldn't matter much, since it has pretty low requirements, I believe.
There's talk of porting WINE to OS X but nothing has come of it yet, that I know of. Even when it does, it will probably be short on optimizations and face the same issues as Bochs.
The other option is to taint your home network with an old PC. Castoffs from the office work particularly well, if you're so lucky. I have an archaic machine that I got from work which fortunately runs PBEM, browsers for testing websites and a few old Microprose turn-based games just fine. It's extra space and annoying to have to actually use a PC but it does work, however. I just like to think of it as my way of reminding myself how much I prefer my mac.
