Scott Baker just posted a video showing a prototype of a controller adapter that allows you to use a PlayStation 2 Dual Shock controller on an Atari 5200: http://www.smbaker.com/atari-5200-playstation-2-dual-shock-controller-adapter
This project stands out, as the adapter retains all the features required for 5200’s (number buttons, etc), allows use of a comfortable PS2 contoller…and the dual-analog sticks can even be used in tandem to play double-stick games like Robotron! Scott has no plans on manufacturing them, but posted plenty of info and schemtics on his blog.
