Hi guys, I was using Avactis shopping cart for some time and I found it awesome. It's not complex at all, yet easily can be customized. I skinned it rather fast. Almost hadn't rewrite PHP code to do what I wanted. Good thing.
There is a good cart called ezypal (.com). Their support is horrible, but the product is really nice. Just don't expect any support from them (they never replied to my questions and that's over 3 weeks ago). Unlike most of the carts out there, this one isn't overloaded and very userfriendly.
Cubecart would be my second choice (there is a free version available too)