Edit - The big thing that bothers me about oscommerce is that the last release was July of 2003, and that one marked as unsupported (being a milestone release). It seems there is still dev going on though. Also the code is a bit of a mess, in my opinion. The good part about it is that it has a huge number of community contributions you can use to add functionality.
Zen cart is also quite popular. From what I've seen, it's basically a much cleaned up version of oscommerce.
PHPShop is another, but I don't think I'd call it better.
