To give you the best answer, you'll need to tell us a little something about how you would normally use Photoshop.
Your post implies that you are not aware that Photoshop was originally a Mac program, and it has always been made for the Mac. So if you're familiar with Photoshop on Windows, the Mac version is available and very similar (mostly just remembering to use "command" for "alt" and "alt" for "Windows key"
)
As a general PS substitute I second chscag's Pixelmator suggestion. I use it every day for quick web work. It's a far cry from Photoshop's power but it does all of the same basic functions with great aplomb (though I wish they'd hurry up with that update, there's a bug in their resizing. They sent me a version to fix the issue but they should update the regular version).
OTOH if your main use of PS is retouching photos I would have different suggestions.