(Not rating because it's probably more accurately classified as a utility than a game.)
Freakin awesome. Small, fast, and free. QuickTime Player can be very picky about playing Flash files -- sometimes they open, sometimes not, and when they do open, sometimes sound doesn't sync or something. (I'm guessing QT uses it's own decoder for SWF, instead of using the plugin? Not sure.)
iSwiff just uses the Macromedia Flash plugin for running flash files, so they work exactly the same as they would in Safari, except that you can scale them to any size that you want. By right-clicking on the window, you can change some settings with the plugin, like quality and zoom. (Tip: If a file is playing back very jerky, just bump the quality back to Medium or Low. Low is a little nasty, but Medium looks fine.) It also allows you to run the files in full screen mode, so it's great for showing off your H*R collection to a bunch of friends. :)
Be sure to check out the link to the Prince of Persia flash game for a great blast from the past.