Can I still weigh in?
I think Flash and .NET are okay. ActionScript and .NET have always had large open source development communities - even if these developers could only work on proprietary platforms (this is no longer true for .NET). The Flixel framework used to develop Canabalt and many, many other games (
Red Rogue is an open source, highly innovative example!) is open source.
ActionScript can be transcompiled by Flex and HaXe, which are both completely open source. HaXe I don't know much about, but I think it transcompiles into JavaScript or something and can do all sorts of funky stuff.
As for Unity, I'd say no. It is too closed. Same goes for GameMaker, though the ENIGMA engine is an open source reimplementation (that is as of yet incomplete).
Also Julius, when talking about projects like OpenMW (or ScummVM, or OpenTTD for that matter), consider that the reimplementations are made of games that were never really meant as complete game engines. The ScummVM team has pulled documentation because they don't want people to make games for an engine that uses loads of hacks to get working (and also, uses an ancient version of Lua for scripting). Being open source is not an afterthought, it's a core aim. However, creating a complete game is not a goal, especially since these engines are not intended for complete games to be created with them.
I agree with Julius' policy on non-libre art.
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