Most of the bigger Open Source projects are well funded and have large communities, usually professional developers using their spare time, doing further development and adding new modules, design conepts and so forth. They do ask you to make a contribution but that is voluntary and you only do so once you can afford it or once you are sure the software you have received from it has done you some good.
One thing many purchasers of proprietary software can not get their heads around is that these projects are not done on a shoestring, they are not built by amateurs and they are continually developed and improved. There are no shareholder maws to feed for the most part, although that is not universally true. Many have sponsoring companies, although what that usually amounts to is a few servers for hosting and development and some limited staff time, in return for which they receive a well earned first bite from many at the support and project development cherry.
Open Source does not necessarily mean free, by the way, although there is no need to wrap it in inverted commas when it is. It actually means, put somewhat simplistically, you have access to the source code so can alter the application to suit your purposes rather than having to accept what the vendor has sold you.
Hope that explains it in outline. To do so fully would take a book.
One thing you can be sure of: if it is genuine Open Source, there is no advertising although you are usually asked to keep an acknowledgement to its developers in the head section (the top of the code which is not seen on screen).
OSCommerce, Zen Cart (which started as a result of a disagreement in development priorities between the original OSCommerce development team), Joomla! and Drupal are all very highly regarded, well supported and well financed projects from the FOSS (
Free
Open
Source
Software) end of the Open Source "community". My preference between the first two is Zen Cart. But that is one of the beauties of OS. Choice. Dave (uk1host) and I can disagree forever about the relative merits of each, but I'm sure having the choices open to us is something we would agree on.
I repeat, they are not paid from ugly advertising on your site so you don't get any.