I've been working on an e-store for a friends site for a couple of weeks and went through many store options, I'd second midwest's endorsement of oscommerce. It's a script you'll have to install yourself and you can find out more information at
http://www.oscommerce.com.
As a bit of a threadjack, I was shocked with how few "lower end" shopping cart options existed. While scripts like osCommerce and zenCommerce are great - they're vastly overpowered for the majority of e-commerce used as "incidental" features of sites (ie: sites that aren't geared exclusively to selling merchandise). I know when I want to add selling options it's often more in the vein of "band wanting to sell cd's and t-shirts" than "clothing retailer looking to carry entire inventory on-line". I finally found this excellent
Client-side javascript shopping cart (halfway down the page) to integrate with paypal, but it's certainly not admin-friendly, and required a week of hair-pulling tweaking to work, but it's rediculously robust with what it let's you calculate and pass to paypal (down to weight based zone shipping calculations).