Email based Messaging (Delta Chat and COI)
There are tons of messengers out their, but they all share the same problem. They are all isolated from each other, so if you want to contact someone via this messenger, the other person has to use this same messenger too. This leads to everyone using the same shitty messenger (in my region this is WhatsApp), although there are many better alternatives. A solution to this problem can be the use of email for messaging.