Paintshop Pro was the image management tool that I used pretty much from day one on Windows - the thumbnailing management was great and it actually sharpens images much more accurately than anything I have tried so far on a Mac.
PhotoGrid does a bad job of thumbnail caching but the good thing about this app over other apps is that it allows you to open and display multiple images at the same time - big deal huh? Well on a Mac it seems to be as this is the only application that I have been able to find so far that does this.