Relative licensing restrictions of Ethereum and other projects- Open Source?
Is Ethereum open source?


Yes. You can find all the source code on Github.

Technically, Ethereum - as in the idea - is open source since you can read the full specification in the yellow paper. 

Ethereum - as in the software - is open source and you can find all major and minor implementations on Github: geth, mist, etc. (GPL, LGPL)


  But then what about the Ethereum Foundation?


Yes. You can find a general statement here