What Are Some Good Examples of Hypermedia APIs?

I'm increasing my coverage of hypermedia APIs in 2014, as we move from discussion to concrete hypermedia implementations in the wild. In support of this, there was a quick conversation on Twitter today regarding some good examples of hypermedia APIs, that I wanted to share with you.

Chris Metfcalf (@chrismetcalf) of Socrata asked:

Providing some much needed examples of hypermedia APIs in the wild. I haven't looked at the details of each implementation, but I trust Darrel's opinion, and will take a deeper look under the hood when I have more time.

It is nice to have three clear examples of hypermedia APIs to showcase:

A hypermedia example from the world of commerce, providing an example that fits nicely into the API economy.
An interesting approach to using hypermedia APIs for discovering and managing your family history.
An enteprise example of hypermdia APIs from the content collaboration platform huddle. 
Amazon AppStream REST API
The Amazon AppStream web service provides APIs you can call to manage applications hosted on Amazon AppStream and to manage client sessions connecting to those applications.
Our platform makes searching AV files simple for developers to integrate into their applications. This means the valuable mindspace and the time of developers can be employed building new and better services that use the information locked up in existing and growing AV libraries. In other words Clarify connects your AV files with their potential.
Microsoft Lync Web Developer
Microsoft’s Unified Communications Web API (UCWA) is the Next Generation Platform for Mobile and Web Development.
One of the key features of the PayPal REST API is HATEOAS (Hypertext As The Engine Of Application State).
VerticalResponse's API generally follows the REST model, based on the principles behind HTTP.

I'll keep tracking on other examples of hypermdia APIs in the wild, and see if we can't begin shedding light on some of the better implementations out there, and see about getting some of these providers to discuss their API developer strategies at @APIStrat or @APIdaysGlobal. If you know of any other examples, please let me know.

UPDATED: 04/17/2014 I added Amazon AppStream REST API, suggested in the Twitter stream by @zdne of @apiaryio.
UPDATED: 04/18/2014 I added OP3Nvoice and Lync Web Developer suggested in the Twitter stream by @darrel_miller.
UPDATED: 05/01/2014 I added Paypal Developer suggested by Pedros Santos on API Craft.
UPDATED: 05/01/2014 I added VerticalResponse suggested by Rob Zazueta (@rzazueta) on API Craft.
UPDATED: 09/29/2014 I updated Op3nVoice to be Clarify and reflect their new branding.