Yo! is probably one of the most exciting and strangest apps of 2014.
What started on April Fools Day was suddenly a company valued at between $5 and $10 million in July and passed a million users just three months after its launch.

So what is Yo!?

The download page describes the app as the following:

The simplest & most efficient communication tool in the world.
Yo is a single-tap zero character communication tool.

So you can subscribe to a list of users and send them a Yo! – for sure the simplest and most efficient communication tool. Wanna say "good morning"? just Yo.
Instead of sending your friends a text message you can now simply send a Yo! – And if you've never heard of Yo! before you probably now think "WTF?" and if you've tried Yo! it is likely that you think "ok, that's fun … but WTF?!"
It took me a while to understand Yo! and maybe the vision behind it.

"Yo is everything and anything, it all depends on you, the recipient and the time of the Yo." (also from the app description)

Think about that you can Yo! with not only people but things. Anything. Think of the services and websites you use. Think of the electronic devises around you.
What could you do if you can send and receive a Yo! from them?!

But before we look into the use cases and problems that could be solved with Yo! we think about notifications for a moment.
With notifications I mean those little push notifications that get sent to your mobile. Those little messages want your attention and notify you of some event that happend. Maybe an e-mail. Maybe some news. Maybe that you left the lights on when you leave your house.
Notifications are the prime time of your mobile device. It's displayed on your home screen and you pretty sure see it.
That is highly valuable for any service or company. It is valueable to be able to notify the user of some event and get her attention and get her to use the service again.

And this is where Yo! comes in.

How can a service directly notify a user on his phone? and How can a user interact with a service by pressing a single button?
Yo provides an answer to that.

Use cases

OK, what use cases can we think of. Have a look at the YOindex. That is an index of applications that integrate with Yo!.
Here are some examples:

• YOMYPACKAGE sends you a YO! when a package has arrived and maybe is ready for pick up. 
• YOYOUTUBE get a YO! when your favorit YouTube star has uploaded a new video.
• FLIGHTYOGURT set a target price for a flight and get notified when it reaches the price.
• STARBUCKSMAP tap and it sends you the location of the nearest startbucks.
• Need a reminder that you have to take some medication? Or that you have taken your medication? – Just Yo!

And this is just a short list of simple applications. Send a Yo to your house to turn of the lights, or send a Yo to track the cups of coffee you drink.
A lot of these things you already can do with IFTT. Have a look at the recipes that integrate with Yo!

The concept of the application is so simple, but your imagination is the limit to what you can do with it.

YO! API
And for us developers the best thing is Yo! has amazingly simple API (suprise! :)
All you need to basically do is to send and receive simple HTTP requests.
We have played for not more than 1 hour with the API and build two little apps:

Yo-to-all broadcasts your Yo! to all subscribers.
https://github.com/bumi/yo-to-all

TODAYILEARNED sends you a daily link with some interesting facts and wikipedia article.
https://github.com/bumi/today-i-learned

I hope this gives you a little bit of understanding of what Yo! is and inspires you to think how you can use or integrate Yo! in your applications.