While there are many fairly specialised mobile apps out there, Yo, which was just launched by Life Before Us, takes the cake for being the narrowest-focused messaging service available on Android and iOS now.
Why? Because Yo can only be used to say "Yo" to your contacts. As you can imagine, it does not even trigger a keyboard when you want to hit a friend with a message, as a touch of a button will do the trick (Life Before Us touts this as a feature, in case you are wondering why the heck I am mentioning it).
I'll leave you with the description:
So, Yo! (Oh, and a bunch of investors raised $1 million (over £586,000) to make Yo happen.)