I've used osCommerce in the past, but it was rather clunky and not really very good for
SEO (Google couldn't seem to find the products). Perhaps the latest version fixes that, but I haven't had an opportunity to try it. However, I have heard very good things about Magento.
Magento is a bit of a resource hog, so I understand, but a dedicated server or VPS should handle it pretty well, so that's not that big an issue. The interface is really quite neat — much better than osCommerce, really — and it places well on Google. I'm actually thinking of using it for a new eCommerce startup that I'm planning.
How about you? Do you have a favorite eCommerce software?