Is there an OS X tool that allows cropping with a particular aspect ratio?

Is there an OS X tool that allows cropping with a particular aspect ratio?

I'm spoiled by FastStone on the PC. I'm looking for decent freeware/shareware for Mac OS X that will allow basic photo editing, particularly something that lets me pick a crop size or aspect ratio by entering in numbers rather than just a freehand crop (I need to make sure it's 8.5"x11" aspect ratio).