Mac OS X has a built-in way to view all of the files in a folder at once using Quick Look. Go to any folder full of pictures, press Command-A to select all of them, and then press Option-Command-Y.
Once the slideshow of the items begins, click the Index Sheet icon (it looks like a group of four squares) to see thumbnails of everything you selected:
From that view, you can click on any item to see it full-screen, or tap Escape twice to exit.
This is a great way to select exactly the file you want without having to open any application first.
(Thanks to Melissa Holt at The Mac Observer for this one!)