FWIW I've recently been working on a job using Ecwid -
www.ecwid.com - it is an ajax hosted thinggy which is quite neat and has a great community round it and their tech support (forums, bugtracker, knowledgebase) is really good. Has a few flaws that were required for my client like minimum amounts on products but I figured out a sort-of-work-around.
It is free for up to 1,000 products but
not SEO friendly on free accounts. If you pay $11 a month they you have API access which makes it SEO friendly.
Key Features (according to them)
Can be easily integrated to any existing site in minutes.
Can be mirrored on many sites at the same time. Add your store to many sites, manage from one place.
Integration with social networks. Run your own store on Facebook, mySpace and many others.
Simple to use and maintain. Both for store owner and for customer.
Lightning fast. New-gen technologies make Ecwid much faster than usual shopping carts despite which hosting service you use.
Seamless upgrades. You just wake up one day and enjoy new features.
