I recently had a chance to install and test WooCommerce for WordPress.  This is a very robust ecommerce shopping cart that integrates right into WordPress, which many sites use for their CMS.

While the plugin works pretty well, there are a few things to watch out for:

• WooCommerce is “free”, but you would likely require some plugins – for example most of the shipping and credit card plugins cost money.  So by the time you are done, your “free” solution might cost $100-$200 in plugins.  This is still a reasonable bargain, but just be aware up front.
• Some themes are WooCommerce compatible, but many are not.  Even so, integration is not too hard, just expect to spend some time / money getting the look of the shopping area customized.

Overall, though, I’m impressed with this plugin.  It’s a nice way to add purchasing functionality to your wordpress-run website.