AskHN: Current state of cross-platform mobile development?

How do current tools for multi-platform development compare? Which tools are available and where's the biggest community?

I'm interested in building a relatively simple app that should run on as many devices as possible. The app shouldn't rely on any of the phone's sensors. I'm especially interested in creating a good User Experience but since iOS, Android and all the other platforms behave quite differently I feel like this can be a deal breaker.

Please share your insights, experience, links and knowledge about developing one app for multiple mobile devices!