XnView is an excellent, free editing tool that performs double-duty as a superb image viewer.
For starters, it has all of the basic editing functions you expect, including tools for resizing, cropping, correcting red-eye, flipping, and rotating.
And that's just the beginning: You can also adjust brightness, contrast, and gamma, convert colour photos to grayscale, and add watermarks.
Useful too is XnView's ability to convert file formats. You can convert files in a batch, and you can even perform automatic editing on photos - such as adding a watermark or cropping - as you convert them.
Though this editor is not as powerful as GIMP, it is far simpler to use, and particularly convenient for quick photo tweaks.