If you are looking for an open source alternative to Corona, as others have noted, the Moai open source game framework is making a lot of progress and a number of experienced game developers are using it. I switched to Moai in since November and it's basically an open source, high performance game engine with Lua.
Moai might not be suitable for a beginner though. I'd stick with Corona Free edition if I was just getting started, then switch to Moai when you are 100% sure you can create a good game.
