Hydra
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Hydra either refers to a mythological many headed creature, or a W3C Community Group that aims to standardize how a web API can advertise its operations and what types of objects it expects/returns, or references to how they are related, paraphrased "Cut off one hydra discussion, Two more shall take its place."[1][2].
Wiki: http://www.w3.org/community/hydra/wiki/Main_Page
Spec Draft: http://www.hydra-cg.com/spec/latest/core/
Examples
No known real world examples of Hydra usage, certainly no selfdogfooding.
Criticism
WSDL Again
The summary goal "aims to standardize how a web API can advertise its operations and what types of objects it expects/returns" is essentially the same as WSDL a failed effort to do the same.
Unclear what about Hydra is different (enough) to cause it to succeed where WSDL failed.