I'm a big fan of PHP. I have recently (1 year ago) picked up a frame work that I'm finding to be extremely easy. Symfony. www.symfony-project.org. Right now it's on version 1.0 but they're getting ready to release 1.1 I do believe. .NET is always a good route to go as well but I'm not much of an IIS fan.
But those obviously, you'd have to program the entire thing yourself. So I'm not sure if thats something you're looking for or not but is the only way I know of being able to add a shopping cart/login system and customizing it w/o having a company do it for you.

