GitHub offers “powerful collaboration, review, and code management for open source and private development projects.”
See Gists to transclude code snippets to the wiki.
See Git for more about Emacs support for git.
org-protocol-github-lines is a package to add “Emacs buttons” to the github webpage (using a Greasemonkey/user script). The button will open the corresponding file in Emacs using org-protocol (provided by OrgMode).