I've just started using PHPCart.net which is cheap, cheerful and does exactly what I want it to. However it wouldn't be suitable for a site where a client with no wen knowledge has to keep it up to date.
I looked at Os-Commerce:
an open-source e-commerce package. It was no good for my use because it is an out-of-the-box e-commerce site and all I wanted was the "add to basket" button and payment processors. But it might well be perfect for your client's site. Adding new products is easy (if a tad tedious) and once set-up maintenance is through an admin interface. It looks pretty naff in its un-customised form but there are plenty of user written modifications for just about everything from the way it looks to adding new postage rates. There are user forums to help too. It uses PHP and MySQL.
A similar open source package is Open for Business:
I haven't looked beyond the blurb and the demo on the web site but it looks slicker and more professional than OsC - but that is purely my opinion based solely on the external looks and the professional appearance of their web site!