Virtual PC or Parallels
You can play on poker rooms that are not made for Macs by purchasing Virtual PC software.  This software will allow you to play online poker rooms that were built for Windows.  I personally use Parallels which allows you to use Windows and Mac OS X without having to turn your computer off.  It can get slow when switching from Windows to Mac if you’re running lots of applications.  Visit the Virtual PC page for more information.  Both options can be costly

Dual Boot “Boot Camp” - Intel Only
In an advancement by Apple you can Dual Boot a Mac to play at Windows based poker rooms or sites that only offer a non-Mac compatible software component.   This is for Intel based Macs only.  All new Macintosh laptops use the Intel processor.