OSCommerce is pretty cool, and open source and has a stack of great features. Templating can be confusing, but after a little research you'll figure it out.

I also used open cart, which is a new project also open source, but did not use any new version of it has undergone major updates since I have used.

Having used alot of ecommerce software, paid and open source, we have switched to magento, which is feature rich compared to many of the other ecommerce software, it does have a big learning curve but once you get into it you will not look back.

You will need to do some tweaks as you would with any other ecommerce software, loads of FREE and purchasable mods available.