www.pixlr.com is a photo editing tool.
It is similar in functionality to Photoshop Express with all of the basic features you would expect - such as brightness and contrast, hue and saturation, crop and rotate.
This is a lightweight, effective and free application - and the package itself is Flash based so requires no installation. Just open up the website, click the link to begin creating and dive straight in.
When you have finished, simply save your work onto your own computer - no registration is needed.