[mdasen]

It looks like this is powered by Sunbird: https://www.sunbirdapp.com.

Another alternative for iMessage is Beeper: https://www.beeper.com/imessage

They both have a waitlist at the moment, but Beeper seems to be opening up to a lot of new people.

[modeless]

I have Beeper. It works well, except that it doesn't support registering your phone number without an iPhone, so you can only use iMessage with an email address. I believe they are working on fixing that now.

[upon_drumhead]

How could they work on fixing that? iMessage currently requires a cell phone to activate a number on the iMessage network. Are you saying they're partnering with Apple to change this? Are they going to sim clone your number?

[modeless]

The entire integration with iMessage is reverse engineered. They need to reverse engineer the phone number registration part to get it working on Android phones.