Picnik - Free Online Image Editing
There have been several occasions where I wanted to upload some photos and images to post online, but I didn't have immediate access to my computer with Photoshop Elements installed on it. I've used Adobe's Photoshop Express a few times, but it feels far to cumbersome for minor edits and it doesn't support common image types like GIF.
More recently, I'm finding Picnik to be a great alternative for editing images from any computer. There's nothing to install, so you don't need to worry about having administrator permissions. Picnik supports JPEG, GIF, and PNG, which are the primary formats I'd work with in a pinch. Features include resize, crop, sharpen, minor color correction, exposure and contrast, and the ability to save out more than one file format when you're done. If you upload images to Facebook, Flickr, MySpace, PhotoBucket, Picasa, Webshots, or FreeWebs, you can edit those images using Picnik. I also have a plug-in for editing images uploaded to Movable Type, which further extends the usefulness of Picnik. There is a $25 version, which is useful if you need to upload many files at a time, but for quick edits on the go, the free version of Picnik will likely be all you ever need. In addition to bailing me out in a pinch, I'm looking at Picnik as a handy way to edit files on a netbook without needing to use precious disk space for a bulky application.
UPDATE: Picnik was acquired by Google and closed down in 2013. Google continues to offer photo editing via their own online photo editing tool.