The main shopping cart used at WAHM Hosting is based off of osCommerce. It has been heavily modified to make it easier to use.
The add-ons available on the osCommerce website (http://addons.oscommerce.com/) can be used with your shopping cart.
The problem with add-ons is that some of them don't work well together. Sometimes, you'll find that the more things you add, the less things tend to play well together. Another problem with the add-ons are that some of them don't work to begin with. Occasionally, you'll come across an add-on on that website that slipped through the cracks and was able to be added when it shouldn't have been.
However, having said all of that, most of the add-ons available through the osCommerce website can be installed on your shopping cart successfully.
I've opened up this topic in the forum for people to discuss the add-ons available through osCommerce. No, I do not have experience with all of the add-ons available. That would be impossible since there are so many of them (I think there are just under 6000 of them at the time I write this).
I hope people can use this forum to discuss the add-ons that they find. Also, I hope anyone who tries to install the add-ons to their website can come here to get help on installing them. Of course, the majority of your questions may already be answered on the osCommerce forum (http://forums.oscommerce.com/) so you may have good luck searching over there as well.