One of the most popular ( if not the best ) 3rd party User Interface in Windows Mobile is still SPB Mobile Shell v3.5. SPB Shell is a highly customizable UI solution that provides a uniform user interface across all the mobile operationg systems: Symbian, Windows Mobile and soon will be on Android.
Having used SPB Mobile Shell since ver 2.0 till 3.5 for my previous WM 6.5 smartphones, I must say the version for Android is nothing but just impressive ( see video below ). This software present price is US $ 29.95 for Windows and Symbian platform. I would be very interested to know what price range SPB will offer for Android as Android apps are generally very much cheaper with launchers pricing at the range of US$2 to US$5 only .
Before its final release , SPB is now requesting for a private beta tester program involving the consumer variant of the software. One can now sign up as a beta tester at : http://www.spb.com/beta . You need to furnish some details about yourself and selection of beta testers is limited.
Windows version screenshot and details : http://www.spb.com/pocketpc-software/mobileshell/screenshots.html