Source: http://lifehacker.com/5920259/croppme-automatically-crops-your-images-online
Cropp.me a handy webapp designed to do just one thing well: crop images in your browser. No need to open up a bulky photo editor if you just want to crop even a bunch of images at once, thanks to Cropp.me's
auto-crop feature.
Using Cropp.me is dead simple. Upload your images (up to five at once), select from the predefined output sizes (avatar, Facebook cover image size, etc.) or define a custom size, and hit the button. The best part is, using clever image analysis algorithms, Cropp.me selects which part of the image to crop when your output size doesn't match the original's aspect ratio. You can always manually adjust the final cropping if you need to.
One difference from other online cropping tools is that Cropp.me can conveniently download all the images at once in a zipped file.