We have recently been experimenting with an eCommerce module for Drupal called Ubercart - http://www.ubercart.org, and have found it to be quite impressive, given it's early stage of development.
Ubercart offers most of the features you will find in other cart systems, including easy category and product creation, various shipping methods, multiple payment methods, etc. Since Ubercart is still in alpha stage, some features that our clients would like are still in progress, but this system seems to be a giant step for the Drupal community.
One important thing that we would like to mention is Ubercart's ability to process orders without requiring user registration. Other famous shopping cart systems, such as Zen Cart, currently require you to hack the core to provide this functionality, and it's nice to see that the developers of Ubercart are designing this software for maximum compatibility.
If you would like to read more about Ubercart, visit http://www.ubercart.org
Final Alpha Version Released
Just a quick post to point out that we're on the last alpha version now (Alpha 8) and expect to go beta in the next few weeks. It's quite exciting, and we appreciate the attention a lot of folks have given us over the last year. For more info on the release, you can check out this blog post.
The skinny... file downloads, expirable role promotions (site memberships), and workflow stuff have all made it into core since Alpha 7e, and I'm sure plenty more has changed since May. ; )