Jacob is right, WP E-commerce is a great shopping cart plugin for Wordpress. But you should have good shopping cart themes (e.g. www.shopperpress.com as well so that it can be integrated properly.

There are other good options as well e.g.eShop wordpress.org shopplugin.net

Also see "10 Powerful Shopping/Ecommerce Plugin Solutions For Wordpress" for in depth review speckyboy.com

I haven't used any yet, but SimpleCart(js) looks the coolest. Here's some others I've bookmarked. The last one ain't free, but it's cheap enough. If you're only selling a few items you can also use PayPal "Add to Cart" and "Buy It Now" buttons.

Www.instinct.co.nz
getshopped.org
shopplugin.net

The WP E-commerce plugin rocks (bit.ly, very customizable, and you can get extra add-ons for a price.

Take a look at this Simple WP Shopping Cart app:SimpleCart(js) is a simple, attractive, and flexible WordPress theme that taps into the power of both Thematic and WP E-commerce. Features include AJAX cart functionality, a grid layout for products, and a flexible color scheme, easy to swap out images and completely customize the theme to your needs.

www.chris-wallace.com