Ask HN: Good mobile open source projects?

Hello HN,

I'm working for a company building their app on iOS, Blackberry OS and Android. To hopefully improve my skill set, I'd like to work on some open source libraries or apps for any of these platforms. Most "apps" seemed to be fairly easy (in a technical sense) to implement once you've got the basics of UI + Communicating with REST APIs down.

Are there any interesting mobile open source projects I'm missing completely ?