Picasa for Mac is a free app from Google for importing, editing, organizing, and sharing photos. The first time you launch it, Picasa can quickly search your hard drive (or select folders) for your existing photos, pulling them into the Picasa interface without actually duplicating them and taking up drive space.
Picasa's interface feels un-Mac-like at first, with a nonstandard scroll bar and even slightly fuzzy fonts--but it's still instantly recognizable (and usable) by anyone familiar with iPhoto, with similar toolbars and three-pane organization with folders and albums. Picasa makes it easy to create slideshows, movies, and collages, and--not surprisingly--the app has tight integration with Google's Blogger and the excellent Picasa Web Albums photo-hosting service (online companion to the desktop app). Picasa has good editing tools as well, which let you try different effects and even easily add text to photos, and it has an intuitive tag system that lets you add batch tags, geotags (using Google Maps), and even name tags using a fairly proficient face-recognition feature (which can tap into your Google Contacts).
Picasa isn't perfect--with flaws that range from its interface idiosyncrasies to an initial setup that's likely to grab many irrelevant images--but this app provides a relatively feature-packed and much cheaper alternative to similar photo-management apps, and it's especially attractive if you're already using many of the Google services it accesses.