Anyone running an Intel Mac (or Linux for that matter) should spend a few quid on getting
CrossOver. I've been using it for about a year and have just installed the Windows version of Photoshop 7 on my MacBook....it runs faster than on any of the PCs in the house, and is seemlessly integrated into the Mac OS without having to have Windows installed.
Apparently games like Half-life (and attendant Mods) will run on Mac and Linux under CrossOver as well.