AFINSA
Offers a country-, theme-specific searchable database of stamps worldwide. Caution: it employs its own catalog numbers, though.

Stamp Images on CD
Available from The Philatelic Computer Study Group's site, the CD boasts a lot of stamp images.

StampFinder
The site where you look for those stamps not yet in your possession. There is a searchable database for stamp offerings (by item or topic), Covers and Specialties (with images).