For a very simple, lightweight utility I like to use
IrfanView. It is basically insta-load and is the quickest way I know to do resize/crop/brightness/contrast of images. It also has batch operations which can be quite useful if processing a lot of images.
For things more intensive, I use Photoshop. However, for 80% of the most common things I need to do I just do quickly with IrfanView or do some prep with it before full editing in Photoshop. Gimp/Paint.net are good free alternatives to Photoshop, but for most basic tasks I find IrfanView quite capable and much faster to use.