I've been reviewing the [terms of use][1] of the API, but I prefer to ask my question straight here in order to be 100% sure.

Is it allowed to create an app that:

  * is paid, which revenue goes to the developer of said app,
  * allows users of said app to donate money to the developer of said app,
  * contains ads which revenue goes to the developer of said app,

or any combination of these?

Second question: does it matter if the app is open or close sourced?

Of course, all this assuming that the app complies with the terms of use, regarding the API attribution, the Stack Exchange trademark and brand guidelines, and the termination of the API.

[1]: http://stackexchange.com/legal/api-terms-of-use