@Arta incorrectly stated, with regard to ZenCart,
They are both old and doesn't update at all.
ZenCart IS maintained, IS updated frequently and DOES have a decent roadmap into the future along with a robust community.
Here is a recent (March 2011) update from the ZenCart Team:
http://www.zen-cart.com/forum/showthread.php?t=175569
Personally, I have installed, configured and maintained no less than 3 ZenCart shops, having found ZenCart subsequent to using "Selena Sol's" and Gunther Birzniek's Perl-based eCommerce app creatively named, "WebStore" for several years.
I thought I'd died and gone to heaven when I found ZenCart because I am not a programmer - I'm just a plugger who pushes hard enough on something to make it work.
With ZenCart, the community help is excellent. The "Getting Started" documentation got me started and got my websites running.
I've looked at Magento (too da*n complicated) and just about every shopping cart that's out there.
Maybe I'm just getting old but things that are shiny and new still do not compare to my OLD ZenCart for the key features I need:
- Secure.
- Integrates with Authorize.net & Paypal, among others.
- TRUE Quantity Pricing.
- Price based on Options.
- Categories.
- Fine, granular control over Product page Title and Keywords.
- Easy to update when updates are released because I don't muck with CORE files (and I've never had to).
- Thriving, responsive community.
- Open-source. TRULY open-source.
I do have one complaint, though. I WISH the folks at ZenCart would reorganize their add-ons page to make it easier for ppl to find an add-on and know when the add-on was last updated. But as you can see, this is not a complaint about the Shopping Cart software itself.
Hope this helps.