I've just gone through that same evaluation process. I actually had the exact opposite criterion: I wanted a game engine with source. I considered commercial en

I've just gone through that same evaluation process. I actually had the exact opposite criterion: I wanted a game engine with source. I considered commercial engines but they almost never come with source or have exorbitant licensing or cost conditions to get it. Based on that, I've been trying out the Moai SDK. It's OSS and uses Lua for scripting. It's primarily 2D focused (though it runs on OpenGL and there are underpinnings of 3D in the code) and runs on iOS, Android, Mac, and Windows. So if you're a Windows developer wanting to do iOS, it's a great option. You still need to do final build/QA/deploy from a Mac, though. The engine appears to be well-written, if idiosyncratic. These guys obviously know what they're doing. Their tagline right now is that it's an OSS game engine for professional game developers. The huge caveat is that the documentation is really lacking. I know it's something they plan on addressing soon, but it's an issue right now. If you want to give it a go, expect to consult the source quite a bit.