You can view .PSD Adobe Photoshop files directly in Mac OS X with no additional software, thanks to Quick Look and Preview.
To get a fast preview of a PSD file, simply select the document within the Finder and hit the Spacebar to launch the PSD file rendered in Mac OS X’s very own Quick Look. If you want a bit more control over viewing the PSD file (zoom, resize, whatever), you can take it a step further by opening the PSD document within the Preview app, just drag and drop it onto the application icon and away you go (note some versions of Mac OS X will automatically open PSD files within Preview if you just double-click on them, assuming Photoshop is not installed on the Mac).
If you want to edit a PSD file with the same type of capabilities as Photoshop without paying for it, you could try using Gimp, a free open source Photoshop clone. It’s not perfect but works pretty well for image editing, and if you’re not trying to do anything overly complex, it’ll save you a few hundred bucks.