Adobe Photoshop CS5 PC version running via boot camp
Parallels is fantastic. You can either install Windows to a Bootcamp partition (so you have the option of booting straight into Windows), or just as a virtual machine. Either way, Parallels can then run in "Coherence Mode", which will allow Photoshop to appear on your Mac desktop as though it were a native application. I use Parallels for developing Windows software and for games, and the performance is brilliant.

Of course, you could talk to Adobe about a crossgrade to the Mac version - if you don't already own Windows the cost may be comparable.