MugTug offers an attractive suite of tools to help you work with graphics online.
Darkroom is an easy-to-use image processor for tweaking an image's brightness, contrast, exposure, contrast and saturation, as well as applying a few special effects.
Sketchpad is an interesting paint and drawing tool, with all the usual options, and some surprising extras: a spirograph drawing tool, customisable gradient and pattern options, a range of swatches, and more.

And once you join the site's online community then you're able to share your creations (and any of your other favourite photos) and view those of other members, add a comment to anything that catches your attention, and share ideas on how to improve your work.