Irfanview. http://www.irfanview.com/
The fastest, and best that I've seen, quick image editing program. I use this daily...primarily for quickly cropping screenshots, putting multiple clips together on the fly, resizing and/or changing the DPI of images to make 'em web-friendly...etc! Awesome program. It can do a lot more, but these are the things I often use it for (daily).