Beeper Mini is an app which allows Android users to send blue-bubbles(iMessages). This was done by reverse-engineering the iMessage protocol. Recently, the app stopped working, however it is back https://blog.beeper.com/p/beeper-mini-is-back

Related question: https://manifold.markets/alice/will-beeper-mini-stop-working-by-th

Disclosure: I did lose money on that question.