Since Mark Shuttleworth said
that moving the Metacity window buttons to the left "opens up the space on the right nicely", lots of button ideas came out. The most controverted one is probably Esfera
, an idea by Pablo Quirós which Mark Shuttleworth forwarded to the Ayatana mailing list.
A recent idea by Izo makes a lot more sense than all the previous ideas. Have a look (it's called "The Workflow Button"):