Silverstripe ( open-source )
A great/fairly new CMS that is very easily extendable for PHP. Voted “most-promising new cms”. Great documentation and support. Easy learning curve. I found this recently and have to say that it is probably my first choice when I need something flexible, fast, and easy for the user. It is not just easy for developers, but the interface is simple and extensive enough for website owners to dive right in and not be totally overwhelmed. Remember what it felt like the first time you opened Photoshop or 3D Studio Max? Your users will not feel like that.
http://www.silverstripe.com
Magento ( open-source )
Voted “Best New Project” by SourceForge.net. This e-commerce tool is very vast and has pretty much anything you can think of. The structure of the code is very detailed and organized. Documentation could be better and learning curve might take a little longer than usual. It is open source, very powerful, and well-supported.
http://www.magentoecommerce.com
PinnacleCart
This is DesertDog Software’s e-commerce utility. They have very intelligent / immediate tech support. This cart is recommended by PayPal. The cost may be a little steep, but if you can afford it, it is worth it. There is a long list of features including: UPS Shipments (not just tracking), extensive reporting, support for many gateways, great discount options, +much more. The structure uses Smarty and is VERY customizeable. Third party plug-ins are limited. No APIs or customization documentation offered.
http://www.pinnaclecart.com