Need to crop or resize a photo but don’t have Photoshop on your computer? Picnik is an easy-to-use web-based photo editor. It allows you to rotate, crop, resize, adjust exposure, and even add text or graphics to your image. Photoshop.com also has a web-based editor that has a lot of the same features, but is a bit more sophisticated. If you want something that is even more advanced and resembles the Photoshop application, try Pixlr. All of these services are free.
UPDATED 9/14/12: Picnik is no longer in service. Instead I highly recommend PicMonkey as a free and easy option.