Picasa is a terrific photo cataloging program from Google which allows you to catalog photos, create labels, manipulate your photos and share them with friends.
Picasa has a Webpage Export feature where you can export selected images into webpage templates for gallery-style viewing, and it will automatically resize the images and create thumbnails for you. You can customize these templates or create new ones, but the scripting is a bit of a pain to work with unless you’re a Perl junkie and are used to working with cryptic code.
Picasa also exports image info to XML which makes the applications for it limitless, and I found a really great PHP gallery script called Picaweb that parses this XML and renders full galleries. Make note that this script is still in beta (v0.3.1 when I got it), and I’ve already fixed a couple bugs while I was customizing the gallery for my use. If you want a copy of my updated code, let me know and I’ll toss it your way.