A Fast, Free Image Viewer: Nexus Image
"Nexus Image (no relation to Google or the Nexus line of phones) is a free, sharp-looking image viewer that loads images quickly, displays EXIF information in a transparent overlay, lets you browse images via keyboard shortcuts, and can even dim your desktop to provide a lightbox effect while you browse."
I've tended to use Picasa as my image viewer, or ACDSee's Quick Viewer if I've got it installed on the computer I'm working on, but this looks like a fast, lightweight solution. I took it for a quick spin, and it's indeed fast and has some decent features. On the plus side, you can use a scroll-wheel or track pack to move through images in a folder. Weirdly though, the keyboard arrow keys won't work if the image window is in focus (you need to click on the column of thumbnails for them to work). Minor gripe though - this looks like a winner to me!