Stardock does have a free program that adds a Start Button called Start8 at
http://www.stardock.com/products/start8/
It wants an email address before it shows a download link but it appears you can leave the address empty.
The current version of Start8 is timebomed just like the Windows 8 CP and will stop working on September 1, 2012.
I would imagine be then, Stardock will have a new version to replace it.
The program by default shows a menu when you click it with all the apps including metro ones listed and you can change the function by right clicking on the button and making it just go to the full screen Metro start screen.
I'm not sure how Stardock could have done it but I think the menu they display just embeds the "All Apps" Metro screen inside a popup window that appears when you click the button.
They also have an option to just display the normal Metro start screen in it's popup with the tiles but inside a small window there is less room to display the metro tiles.