I stumbled across this game for teaching lambda calculus last week: http://worrydream.com/AlligatorEggs/ —and thought it'd be cool to see it executing through software, so I spent a few days building an app to do that.

You can try it here: http://symbolflux.com/lambda-gator/

It's very basic at this point (it will lock up on syntax errors! see limitations in the "ABOUT / INSTRUCTIONS" dialog), but the source is available under MIT license (https://github.com/westoncb/lambda-gator), so maybe it will be of interest to someone.

I also made a video describing how to interpret the visuals etc.: https://www.youtube.com/watch?v=Uiv4bKRZXek