Seeing all those services which do sth per git push let me think that github might create dev app marketplace...

This sort of already exists through the service hooks they have, there are 122 services atm.

Good point but improving the presentation there could be great for both GitHub & its users.

I agree, a nice shiny app store with icons could be a great feature for Github users and developers using their APIs. Right now service hooks are pretty buried in the settings panel.