Radakan's devs have decided to use the jME aka jMonkeyEngine, just like JCRPG does! Neat! I admit that I was expecting the project to evaporate, it just had too pretty a website and some team member was very concerned with picking the right wiki and installing a flash audio player on the site, while there was nothing of the game in sight. (I also was afraid the team would come up with some idea like "Hey, let's make our own 3D engine! Right?")
I'm glad I seem to be wrong, the first tech demo video has been uploaded. (I'm not sure if that's not just some engine-bundled terrain and model, but even if it is, what the hell! They're grappling with it! Enough proof of interest and engagement for me!) I just hope they won't let the plans for game story get in the development's way. But I might be wrong worrying about that - Radakan is a "single-player sandbox 3D RPG". Sounds like something that is free of the bug feature "storyline" and like something one can have fun with, without touching any code. Yay for spawning 1k melons mid-air and abusing rag doll physics!
I tried to find an open source 2D adventure game engine and also was given some recommendation in the last post's comments. Wintermute Engine (WME) and Adventure Game Studio (AGS) caught my attention, both are closed-source freeware. What surprised me, was that AGS has been ported to Linux and MacOSX! Thanks to .Net and Mono (the open source .Net implementation) I suppose.
I visited WME's IRC channel and asked Mnemonic (the only developer, since the beginning of the project) about his views on open sourcing the engine. He said approximately "not now and not anytime soon". These are his reasons: Leading an open source project means stress, because people whine about your code. It requires work, because the code has to be put in a readable state for others. It requires even more work, because submitted patches have to be tested, to make sure that they don't 'feature' new bugs.
"Simply dumping the source on SF.net" was right out, a decision which seems logical. After all, why should you make a project open-source, if you're not able to manage foreign development to take place in it? But on the other hand: If one has not enough time for maintaining an open project, maybe "dumping" the code isn't so bad a thing? In the end it shouldn't do any harm to the project and if someone decides to fork it, it can only be good for the gene pool.
I haven't talked to anyone in the AGS team. I hope I'll get round to catching up on that.
Update: (thanks to the comments) There is only a client, no editor of AGS for linux. Also there is Adventure Game Goddess, something about being an open source adventure game engine. I don't know, read it yourself ;).