I use osCommerce, and here's why:
osCommerce is free and it's open source being one of the most popular shoppingcart solution on the net. it's the most flexible i've seen and the list of features and contributions are the size of a thick text book, meaning you can do it all with it.
The amount of support you can get is unlimited. oscommerce forums are active. almost every experienced web designer has heard of it and can do anything with it. you aren't at the mercy of one provider, anyone can make changes for you, fix issues, and get you going.
The designs and templates are also endless. You aren't at the mercy of one template shop. you can get a custom design done for dirt cheap and there are millions of templates available for easy installation. The cart itself is also user friendly.
When it comes to payment integration, it can't get easier. You can add google checkout, paypal, authorize.net, or any other payment processor. payments go through smoothly and it seems easier then anything else out there.
It can handle unlimited products, you can add tens of thousands of products seemlessly. it takes about three minutes or less to get a product in there with full details, specs, pictures, the whole enchilada. seriously, anybody can do it.
Everything is managed through one admin panel, literally everything. No need to go to ten different sites to get organized. Everything is in one easy spot which again is very user friendly. There are lots of SEO contributions, which make your site easy to find in the search engines. They are easy to install and fix up your site automatically.
Hopefully this helps all of you who are still in a search for a shopping cart solution. take my word for it, go with oscommerce.
