osCommerce is a PHP based online shop that is distributed under an open source software license (GPL). Remarkable is the multitude of so called contributions (extensions) that can be used to add new features to the shop. Another positive aspect is the availability of many different translations of osCommerce.
osCommerce is definitely on par with (or even better than) many commercial products. Unfortunately the source code quality is worse than most software I ever saw. PHP code is mixed with the html source everywhere. A template system is not part of the default distribution (version 2.2 milestone 2). Although the developers plan to a template for milestone 3 they don't want to use one of the established systems (e.g. Smarty).
There is no real plugin interface (with the exception of some tiny subsystems) so that every change must be heavily tested. Security updates and bigger layout changes will cause enormous efforts and must be repeated with every new release.
Finally: English is not my native tongue. Please feel free to correct any errors you find. Thank you in advance.