Hello I build sites fulltime.
There are two I would suggest which are paid versions.
The first is Viart. It is brilliant software and very powerful with a super fast search engine.
www.viart.com
The second is Avactis.
www.avactis.com
Personally I have spent countless hours investigating and playing with carts and those two above in my opinion are the best.
If you want a free cart you can take a look at Magento -
Magento - Home - Open Source eCommerce Evolved
Good luck with what you decide.
Joe