The most prevalant rails one is Spree:
http://spreecommerce.comI'm in the opening stages of building a spree site right now, and at this early point it seems really good. The developers are responsive on irc and on the list, and the repo gets regular pushes.
Its a rails3 engine, and makes full use of this and other rails3 features, plus the devs have extensive plans to use the various 3.1 improvements. It's really modular; the spree gem is actually a meta-gem with deps pointing to other gems, 'spree-core', 'spree-auth', 'spree-promo' etc.
You make all your customisations to it in the same way, by decorating the various core classes, following the same pattern as the spree submodules themselves.
Oh and it has shitloads of tests, both rspec and cucumber/capybara.
Basically it seems pretty sweet.