I'm afraid that your options are extremely limited if you want the software to be free. Picassa was already mentioned and the only thing available which is free for now.
If you're willing to pay, depending on what you want out of photo managing software, there are a slew of options out there. If you're not a pro, and want to go a level higher than iPhoto, you might check out Aperture. There's a free 30 day trial up on the Apple website right now.
I personally use Adobe's Lightroom 2 (soon to be 3 with major new features) but downloaded the Aperture trial just for kicks to see if it has made any new leaps since the first time I tried it (and didn't like it).
While there are a lot of new features (they note 200) I still don't like it nearly as much as Lightroom for several reasons. The most important reason being: it's an insane resource hog when it comes to importing and processing photos.
The other day I went to import almost half of my photo library (about 8k photos) which consisted solely of RAW files. Long story short, I could not do a single other thing while the importing and processing was going on. It took up more than 160% of my cpu cycles and more than half my physical RAM. It wasn't so much the importing, but the processing which I think consists of creating thumbnails for all the photos. And this is an absolutely necessary thing for it to do. It was also processing faces for its face recognition, but I wound up turning that feature off, and yet it sill ate up all of my MacBook Pro's resources.
Lightroom has never been this way under any circumstances. Outside of that, however, I think the GUI is very nice and clean, easy to navigate and is very intuitive. BTW, Lightroom also has a free trial for its beta 3 version.
Try both, but also try and get an understanding of how each of them works because the workflow is totally different for each. I use Lightroom in a fashion which allows me to utilize folders exactly as they are in Finder, as well as by keywords, full metadata (automatically shows dates in a different pane) and by collections. So I get the best of everything. I'm not sure if Aperture does folders as they're seen in Finder, but it might.
Outside of these, I really like Photo Mechanic, but it's not free either.
Google will also yield results under "photo management OS X"
Doug