Update: I have finally switched over from MonsterCommerce to OSCommerce. Over 11,000 products. I am quite pleased with the results, to date, though it is early. My next step is to integrate a QuickBooks export module into the system.
I find zencart pretty good. Has a lot of extra modules available for free, and has a lot of features similar to paid shopping carts.
I liked OsCommerce besides the fact that skinning/templating it was nearly impossible without a full re-install.
I am designing my website around oscommerce. I'm a PHP developer so i'm still feeling things out as it is the first time i've used it. It does, by default, use tables within the design and a lot of the code is bloated and not done very well. However, it is a powerful script and a good base for any ecommerce store in my opinion. I'm still going to use it, but i'm going to make sure that my front pages are all done using valid XHTML/CSS and properly optomized for search engines. It's just something to think about. I'm not sure how the other platforms measure up in terms of SEO. I wanted to stick with OSCommerce because of its popularity for modules and merchant accounts.
you should try using pharaohcart.com its not free but very affordable. ecommerce sites can be tricky to set up. and when you use free carts it takes much skill. --------------------------------
CRE Loaded B2B is the best....and I've tried about 15 different shopping carts. If you are looking for a free version of this cart consider the rebranded "273Loaded": http://www.273loaded.com/
Update on OSCommerce: I have almost 12,000 products listed and they seem to be working. It seems to have most of the functionality that I was getting from MonsterCommerce (never use that one...PM for details). The only downside, so far, is that a lot of the website cannot be modified/tweaked through the admin panel. You need to go into the code, so if you are not technical (like me) you will need to have a programmer do quite a bit of the stuff for you. But, all in all, I am happy so far. My next task is to get the QuickBooks interface installed and working.