If you are coming from PC world and used to use ACDSEE, which can view images in thumbnails, now in Leopard, Preview.app is a real powerhouse.
When you are in folder containing pictures, just select all (Command + a) or just drag them all. Press Command + o.
Preview.app can view JPEG, TIFF, PICT, GIF, BMP, PNG, SGI, TGA, EPS, PDF and can even play animated gif, too!
Preview.app can crop, rotate, adjust color, adjust size. For those who need to see lens setting of the camera, ISO setting, focus mode, the flash was on/off, now you can do that in Preview.app!
And one more thing, did you know that Preview.app can now cutting people out of the background?
Here is how, open an image > click ‘Select’ > ‘Instant Alpa’ shown in the images on the left. Now here is the best part, just click the background area that you want to eliminate. Click and drag to eliminate more area when needed. When you are done, just press Enter/Return.
Choose File > Save as if you don’t want to replace the original image 