I have recently started using DevExpress' MVVM WPF Framework.
I really like it. The amount of code you get to skip vs regular MVVM is huge. For example (I use the POCO ViewModel based system), to add a command just add a public method. Need it be on/off based on some conditions? Just add a bool method with Can*CommandName* to your class.
Behind the scenes it generates a new class with everything it needs.
This applies to many other things, including a bunch of services that are very easy to use including, including reporting, file and folder dialogs, wizards, navigation and much more.
It is also free (without support), or included with support on their WPF components (which are pretty nice too).
There are a couple of videos that show some of the basics and the nice advantages:
Please note that so far I have not used other WPF MVVM frameworks other than plain MVVM, so I can't compare them to DevExpress.