I'm using VP-ASP right now.
It's a great free cart and I've tried several. If you've got any knowledge of VB script and .asp, you'll have great fun cusomizing it to suit your needs.
Although the basic default layout is somewhat cusomizable, with a little effort, you can do some pretty amazing stuff.
If you need more than just the basic functionality, I'd recommend looking at a few of the option packs.
Visit their website and check out the customer support forums for more info.
http://www.vpasp.com