DOI

 DOI  is an abbreviation for Digital Object Identifier, "a character string used to uniquely identify an object such as an electronic document", kind of like a URL with more bureaucracy.

Tools

 * indiebookclub supports DOI when posting read posts.

URLs work fine
DOIs are unnecessary because URLs work fine, and in many ways are better than DOIs. See additional criticisms:

Organizational bottleneck
"Before Zenodo can issue a DOI for your …" - say what? I have to get some ORG to issue URLs for me? No thanks. Using a personal domain I can issue whatever/however many URLs I want without asking anyone's permission.

Use of minting a red flag
Anyone that talks about "Minting a …" is probably doing it wrong and trying to sell you some service for doing the so-called "minting".

Unobvious time waster
There's so much architecture astronomy talk in nearly any positive description of DOIs that you could easily waste hours of your time before it was clear it was a waste of time. Hence criticism documented here to hopefully help you shortcut such distraction.

GitHub
As of ~ 2014-05, GitHub posted about Making Your Code Citable using DOIs.