You're talking about iMessage. Messages uses iMessage AND SMS so it is a tool I can communicate with everyone without them needing to install anything or have a

You're talking about iMessage. Messages uses iMessage AND SMS so it is a tool I can communicate with everyone without them needing to install anything or have any kind of account. The ubiquity and integration with the OS is unlike anything anywhere else. Every time I've tried to use Linux/Windows/Android it's the killer feature that brings me back.