I cannot give you a complete analysis, but I will offer what I know. I have personally worked with three different shopping carts (when I say worked with, I mean had to get in to the code and make modifications). These are
- osCommerce[^]
- PHP - Open Source - osCommerce has been around a long time. The project/product itself is fairly bare-bones, but there is a huge amount of addons[^] for it. However, just about all addons require going in and changing the code. This is usually pretty easy as the addon "package" typically includes instructions on where and how to make code changes, but it does present a problem if you upgrade to a newer version of the base application. This appears to be somewhat mitigated by the fact that development on it appears to have slowed down. The last release was on January 30th, 2008.* Zen Cart[^]
- PHP - Open Source - Zen Cart is to osCommerce as Subversion is to CVS (if that makes any sense). Some years back a group of people took a drop of osCommerce sources and set out to build something "better". It has a built-in mechanism for installing addons, which means less coding on your part, but it also has a smaller (though still 500+) pool of addons[^].* AbleCommerce[^]
- C# - Commercial - AbleCommerce is a fairly robust platform for eCommerce. With a purchase you get all of the source code for the main shopping cart site, but not for their core functionality libraries, primarily CommerceBuilder.dll. It takes some getting acclimated to where everything is in the code and how to properly hook things, but in general it is not too difficult.
All three of these have fairly active community forums where you can search for answers to questions and typically find them without having to post your own inquiry. Beyond these, Wikipedia has an article