OS X Software: PicturePop

Let's say there is a transparent gif file on your desktop, and you need to create a valid image tag for it in your html doc. You could open it in Preview, but that doesn't give you the dimensions. So you laboriously drag and drop the gif file onto Photoshop, wait, wait, wait for it to load, and then select "Image Size." No thanks. Shave time off your workflow with PicturePop - a free contextual menu plug-in for OS X. Right (or control) click on a file, select "Show with PicturePop," and immediately view your image with dimensions. If your image is a transparent gif, type 1-5 on the keyboard to test varying background settings. The plug-in would be perfect if it displayed dimensions without actually opening the image, but for now it's good enough.

Update: Or if you don't mind a little extra clutter, turn on "Show Item Info" under "Show View Options" in your View menu for image dimensions alongside the file name.