VMWare Fusion is an application for the Mac that lets you run Windows as a virtual machine. No need to reboot into Windows to play games, you can just startup Fusion and run any Windows app right from your comfy Mac OS desktop.
They've launched a new public beta (version 2), which you can download and try out for free. It's beta software, so it's not for critical stuff, but should work fine if you're curious about the software.
I gave it a go on my work machine. Installation was easy, and it automatically detected the Boot Camp partition I've been using to run Windows with. Starting the Windows machine was easy, and it felt prety responsive.
...Until I tried to run a game, that is. I started up Fable, a not-too-recent game with low graphics requirements. After a few screen hiccups, the game started, but Fusion warned me that "some shaders are not supported, and some elements may not display."
I clicked past the warning and started a game anyway. Lo and behold the missing shaders were: me! The main character displayed as just a pair of eyeballs floating in space. Really creepy, and kind of a deal-breaker for me.
So I shut it down. Or rather, tried to shut it down, but had to force quit and reboot my Mac to regain control of the machine.
It didn't perform well for me, but I'd still suggest giving the free beta a shot. Who knows? You may find it does everything *you* need it to do, and be worth picking up a copy of Fusion 2.0 when it comes out.