There is an app on the Google Play store that claims to offer iMessage functionality to Android users. The app comes from a developer named Daniel Zweigart . Apple typically isn't forgiving and I am fairly confident Cuppertino lawyers are gearing up as you read this.
Some users on Twitter claim the app actually works.
WORD OF WARNING - There is a very strong possibility that this app is harvesting your Apple ID and password. I recommend not using this with any Apple ID tied to a credit card.
You can download it here.
Expect this app to get pulled and Apple to find a way to kill it technically quickly.