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"):
Izo is the same guy who came with the amazing idea of integrating Zeitgeist into Nautilus, but unfortunately he's not a programmer so his mockups are just... images, if or until someone tries to implement his ideas. Hopefully someone @ Ayatana sees this and takes it into consideration as it seems (at least in my opinion) something far more useful than Esfera - and here I'm also taking into consideration the "n00b factor" because Esfera can produce some serious confustion among new Ubuntu users.
Izo gives more details on his idea @ http://www.design-by-izo.com/2010/03/29/workflow-and-upload-two-new-button-concepts-for-lucid/