"With the GL Image Browser, you can open a folder on your computer and look for all the pictures that are inside, even in subdirectories if you want. It is no iPhoto or Picasa clone, GL Image Browser is a tool to quickly browse through thousands of pictures"
In some way, GL Image Browser enables me to do much of what I do with Photo Mechanic - not as complete, and not as fast, but still it is a better photo viewer than what you get from OS X Preview app.
Best of all - it is free! Get it from Geekologic.com.