I have heard osCommerce but lemme know what you think! and what about zen cart? what is your favorite!? I need: extreme customization advanced payment solutions ease of use good community multiple themes and add-ons FREE thanks everyone!
I run my gamepacks.org store with http://www.ezypal.com. This is designed for software download and it's only $25.
I would seconded Oscommerce, it has a lot of mods and you can basically skinned it to whatever you like. There is also a lot of template makers for oscommerce making it easier to get templates.
One thing about ezypal is that the owner doesn't really provide much support. If that's important to you than you should definitely look at Oscommerce. What is nice about ezypal is that it's pretty easy to setup and adding new products is super easy. It does have tons of features for a small price.
I've been using osCommerce for almost 5 years, and I've had very few complaints. The community support for the software is fantastic, and there are a lot of free contributions you can add to the basic software. It helps if you know a little PHP/mySQL, but I know there are plenty of people with little-to-no back-end coding experience who use the software.
I actually use a different one but it isnt free. there is only one ad on this page Shopping Cart Software so i am sure you can find it (on the right) I found them when google checkout was introduced. They were the cheapest one that wasnt free that google had verified as being fully integrated. I have yet to have to do any additional coding which has made the $150 worth it. USPS, UPS, Authorize net, paypal, checkout, etc. They even have skins for templates or it is easy to modify header and footer to make an entire site easily from their code. Their documentation beats any of the freebies which has also come in handy. I emailed support twice and both times replies came back within a few hours. I have used the freebies and osCommerce was the best of those but I still like this better..but who am i?