The FastStone Image Viewer is a good find for a multipurpose image tool. Although the main interface looks too cluttered with buttons at first glance, the three main windows are intuitive enough once get your bearings and several included skins make it easy on the eyes.
Clicking a thumbnail puts a larger view of the shot in the lower-left pane, and you zoom in simply by dragging the mouse around. You can resize, crop, remove red-eye, and adjust color, brightness, and contrast. You'll also find a few simple effects and filters, including Lens, Waves, and Morphs. Other nifty features include multiple folder slide shows, batch cropping and batch conversion. Though its image-editing tools can't compete with those offered by a full-featured suite, there's still enough here to make a decent amount of tweaks to your shots, and fast.