I use Mal's Ecommerce,
Mal's e-commerce - A FREE Shopping Cart Service. I like the flexibility it offers in customizing the cart to fit my needs or those of my clients. I also like that it doesn't hinder my SEO efforts. However if you have a large number of products a database driven cart may be your best bet.
On another note Practical Ecommerce has been running a "cart of the week" series on different shopping carts and their features which you may find helpful. The latest is on Volusion. You can check it out here,
Cart of the Week: Volusion | Practical eCommerce. You'll find links to the rest of the series on that same page under Related Articles at the bottom.