I like Preview a lot because it is just so much faster to use than any other image editing app like Pixelmator or Photoshop. Of course, with Leopard, I use it a lot less because of QuickLook. If you are looking for something that has more features or if you are still using Tiger and don't have QuickLook, JustLooking can be a good alternative. A great feature it has is that it lets you browse through other pictures that are in same folder. More info and download link here on MacUpdate.