if you're happy to put your hand in your pocket, and your requirements are fairly simple and you're using some other cms to manage your site, check out foxycart.com. If you want a free open source all-in-one plug and play eCom solution then go get excited about Magento. Both are runny at the nose with standards, seo and semantics. However, don't be premature with Magento. Watch the screencasts and download/experiment by all means. They are in public beta and are advising against commercial development until they make an official 1.0 release.
EDIT: Ahh - text book error, I'm replying without reading! Are you looking for examples or solutions?