Yo the App
Yo, now defunct, let you send the word "Yo" to anyone who was in your contacts list.
The Yo API can be used to send a "yo" to all subscribers of a Yo account.
Users are subscribed by sending a "yo" to the account.
Some example use cases: A blog can Yo the readers whenever a new post is published. Imagine getting a Yo From PRODUCTHUNT.
From the Yo Developer API doc 
Yo in the IndieWeb
A Yo is a way to catch someone's attention. You may want to send a Yo from your personal website. A yo is analogous to "hello" or the old Facebook "poke" feature.
A yo may also be sent in jest as a response to a piece of internet history, the Yo app.
capjamesg published a Yo on his website that linked to Kevin Marks' website. The post was sent as a webmention so the Yo would be received by Kevin.
Here is the Yo on James' blog:
Add yourself here… (see this for more details)