Hi William,
I have been using Cube Cart for the last 2-3 years. ( midwestskatesupply.com)
I do have a few enhancements on my site such as a template(skin) and a few other things. Some were free and some were $. But the ones the were $ were VERY reasonable in my opinion.
They have both free version and paid. Paid is for license and support. However they have a VERY good community that is very good about help with any problems or questions.
Another free cart solution might be Paypals shopping cart. Simple HTML you can intigrate into your website.
For carts you buy, I suggest either CSCart Or CRE Loaded.
I believe both are based on the oscommerce platform, which has been around a long time. Both of these carts are high in user rankings.
OScommerce is another freeby, but in my opinion ugly in its stock form. But if you have the knowledge, is customizable.
Another freeby that is widely used is zencart, but I have no experience with them.
Hope this helps a little.
Midwest