Finally a superb freeware viewer!.
I used them all, ACDSee (not free), and comes in big bundles like the Nero package, IrfanView (I don't like the cheap interface) and XNView. I used the last one for several months, its great. To use shell-extension though, you had to install an extra plugin for XNView. I now tried ALSee, and de-installed XNVIew. I think it's a great fast picture viewer!
Pros: Freeware, fast, good interface, even displays PSD-format, contains several tools in an intuitive interface
Cons: I can't think of one, or it must be the bit ugly Smileware banner on the right top?