Not many know about the Slideshow feature in Preview.app, but it's pretty sweet. Try it out: open up a bunch of images (perhaps a folder) in Preview and then go to the View menu and select "Slideshow" (the keyboard shortcut is Shift-Command-F).

Once in there, you can fit the images to screen and add it to iPhoto, but the best part is the "Index Sheet" (This also has a keyboard shortcut -- "i"). This action makes preview a good lightweight alternative to iPhoto for browsing photos.

For Quicksilver fans, there is a Slideshow action plugin that makes this action accessible without entering Preview at all. Very useful.