Apple has shut down Beeper Mini, the app that brought iMessage functionality to Android. However, whatever techniques that Beeper was using on their app, have now been patched by Apple. While Beeper developers seem to continue working on bringing back the iMessage functionality to their Android app, it does not seem likely for Beeper Mini to start working again.
Beeper Mini utilizes research from reverse-engineering the iMessage protocol. With this, iMessage for Android is easy to start using - simply download the Beeper Mini app and get started. Moreover, the Beeper developers are being quite forthcoming with how their technology works. No servers, or middlemen are required, to make iMessage for Android possible. Beeper Mini is available on Play Store right now, however it is a paid app. It costs $1.99 per month to use, with a free 7 day trial.