My brother mailed me about a Windows theme to make it look like Aqua: http://www.flyakiteosx.com/. I prefer this one, though: Litestep, though I'm too scared to use it. It'll probably mess up my Windows box horribly and I'll need to reinstall and deal with activation foo.
Searching around, I found an Nextstep theme for OS X: Next 2.0. I think I'll try installing it next week.
I use Window Maker on FreeBSD/Linux, have done so for almost a decade. I'm unlikely to switch to anything else.