I want to recommend a somewhat unknown but IMO very good program for organizing photos/images.
It's called PhoA (acronym for 'Photo Album') and can be found here: http://www.dk-soft.org/
It is a purely for organizing images and does not contain image editing tools which is best left for Photofiltre or The Gimp (when using free software) anyways.
Phoa's strength is the multiple ways to organize/view images. You can organize them into groups (independent of the hard drive's folder structures) and asign keywords + other metadata to images.
But the most powerfull feature IMO is that you can create views (virtual groups) on top of your albums to allow many different ways to organize/display the images (without actually moving them around in the album) based on keywords, paths, time etc.
There is a couple of other reasons that I prefer PhoA instead of Picasa. One is that all album data (thumbnails and metadata) is saved into one file. Another is that I can create a photo album of a cd or dvd without actually copying all the images to my hard drive.
Finally a hint for getting better looking thumbnails in albums since by default the thumbnail quality is pretty low. Before adding any images to an album edit the photo album properties (menu: Edit --> Edit...) and move the thumbnail quality farther to the right - I prefer around setting 8 seing the scala going from 0 to 10.