commons

A  commons  is a centralized content hosting website, where people can freely contribute content, often using external web-based identity like IndieAuth, collaborate with others, and easily retrieve their own or shared content with little or no restrictions on importing or exporting.

Commons are characterized by the following:
 * run by non-profits or informal communities of individuals
 * often allow using external web-based identity for sign-in (e.g. web sign-in or IndieAuth, or OpenID on some older sites)
 * allow posting of some kind of content (text, images, etc.)
 * a liberal (or no explicit) terms of service (TOS)
 * little or no claim of ownership of any content contributed
 * require contributions be licensed with a standard open content license (CC0 / public domain, CC-BY, MIT, GPL, etc.)
 * little or no restriction on import/export content contributed, or content about such content (e.g. comments, tags)

In contrast, see: silos

Popular Commons
Many IndieWeb community members contribute freely to various commons such as:
 * IndieWeb (this wiki)
 * microformats.org
 * OSM
 * WhatWG.org
 * Wikipedia
 * Internet Archive

Almost commons. There are some sites which are almost commons, yet have some restrictions/barriers that make them not so. What's particularly interesting about them is that they may be exploring particularly indieweb friendly business-models.

Alternatives to github (likely belongs on Github page)
 * Github, satisfies the most important philosophical reasons for having commons, passes the cloning test below, but
 * has a more restrictive silo-like TOS
 * silo-specific login
 * hard to mirror repository and organizational settings from github
 * hard to get the comments out of github
 * meta data that github collects around the git repositories is siloed
 * that meta data are the data that make github usable and worthwile
 * http://gitlab.org/
 * gitorious

Test
Here is a fairly simple test to determine whether a centralized content site is a commons or a silo:

Can someone freely create a mirror of the site, and if the original site shuts down, have the mirror effectively replace the original from a community perspective?

With a commons the answer would be yes.

With silos the answer is no, typically due to the owner of the silo actively (or passively through threats or perceived fear of) blocking the creation of any mirror, and certainly any mirror intended to replace/supplant the existing silo "community".