One of the more ambitious projects ever to see a beta release - Wine - runs on the Mac. With caveats - it's a pain in the arse to get working.

1. Remove Apple's X11 if you want any chance of using 3D.
2. Get Fink and install x.org 6.8.2 and applex11tools.
3. Install FreeType and FontForge by your favourite means (I suggest MacPorts).
4. Download the Wine source - you could use the Darwine Mac OS X wrapper for Mac-ish goodness, but you'll need to hack the script to do the configure change below.
5. Run configure for Wine using LDFLAGS='-framework CoreServices' ./configure --prefix=/usr/local (or where ever you so wish). The link to CoreServices is needed to stop one of the Font Tools failing.
6. Configure should complete without warnings, otherwise panic.
7. Run make depend && make && sudo make install
8. Finally, run using DYLD_LIBRARY_PATH=/opt/local/lib wine <program> - if you installed FreeType elsewhere then substitute that location for /opt/local/lib. Even better, encapsulate this part in a script.

At this point you should have a Wine installation that runs Steam. Wasn't that simple?