ZenMojo.com

Here is a great, open source, flash photo gallery creation program that works directly with picasa:
Applejuice

And there is also a script that will create the gallery on-the-fly on the server:
Applejuice Build Gallery

EDIT: If you just want to add a little pop to your existing html photo gallery, there is a great javascript called Lightbox.

Also, here is a guide to a bunch of Lightboxes
and Smooth Gallery

Here are some plugins for Picasa to make galleries:
http://webscratches.com/2007/04/03/slideshow-generators-plugins-for-picasa/

http://galri.free.fr/

http://www.paulvanroekel.nl/picasa/