To my eyes, the most promising open source e-Commerce platforms I've seen lately are Ubercart and Magento. We're actually currently using Ubercart here as I write this, for this very site! Ubercart is actually a module for Drupal (a powerful content managment system). Drupal has another e-Commerce module... Drupal e-Commerce (yes, not very imaginitive, I know), but it's not currently quite as full featured as Ubercart.
Magento seems to be very promising, and is currently released in version 1.0. It seems to be built for one purpose, but does it very well! Selling physical goods. It currently doesn't support selling of digital goods (downloads), but there is word of it supporting digital goods at some point in the future. Ubercart on the other hand, is currently at a very stable 1.0 RC2 phase, but has downloadable goods out of the box! This along with other Drupal features and available modules put Ubercart ahead for some people.
One nice thing about Magento though, is that there are many screencast tutorial videos for it. I haven't seen quite so many video tutorials for Ubercart. Magneto seems to be rather complex to theme, but has a very nice look out of the box. Ubercart on the other hand isn't quite as "pretty" out of the box, but thanks to Drupal, is ultra configurable!
What's your take on Ubercart vs. Magento? Leave some comments and let me know!