Tried all of them so far cubecart and Opencart are easy to use. http://www.cubecart.com/site/home/ http://www.opencart.com/
Never really understood why people who want to have an online store (=make money) look for a free shopping cart system (=don't want to spend money first). Neither of the above is appropriate for a serious project IMHO. I always prefer a good stable paid solution. My favourite is Avactis shopping cart. It ain't too expensive though has all the nesessary features and great support that can reprogram everything if needed.
magento is free! If you want a paid cart look at cs-cart.com but I just found the best cart out there for php click below http://www.airvaecommerce.com/ its $1,200 but looks to be well worth it!
Anybody actually used magento, I'd love to know the opinion about it from a heavy user... I'm a big fan of zencart and oscommerce, hoping magento destroys their flaws.