In our game, you can usually quantify success by observing not only your own growth, but also what affects you are having on the greater community and those around you. Judging by that, I'm glad to report that one of my favorite open source projects, Magento, are well on their way to achieving success--if they haven't achieved it already.
Since their 0.6 release which is not too long ago, they've already reached 500,000 downloads. Their user community is also over 44,000 strong and they have over 60, thats right 60, translations. It is a true testament to the Magento team and their philosophy and dedication to the community model and to Open Source.
As if that doesn't suffice, others in the eCommerce space have begun to open source their offerings as they feel the heat from Magento. Oxid, a very popular German eCommerce software producer recently announced Oxid eShop is going open with eShop Community Edition. (Link is in German)
I stand by what I said months ago, Magento is killing it.
UPDATE: As I write someone showed me that Matt Asay posted about this as well on his Open Road blog.