FixPicture is both an HTML and Flash-based tool for quick image manipulation when all you've got is a web browser and an picture in need of some work.
The Flash version is a much richer experience, letting you alter hue, gamma, contrast and more, plus you can add effects like sharpening, auto levels and sepia. You can add images from your computer or from the web (via URL). When you've finished editing, you just save your image and you're done.
The HTML version isn't quite as feature rich but is quick and simple. If you need to quickly resize an image and auto-adjust levels, FixPicture can handle that easily. While it's not going to replace Photoshop, if you need to do some minor image adjustments in a hurry it's a handy tool.