Bitbucket icon
Author: Guilherme Lima
Source: http://www.softicons.com/social-media-icons/flat-gradient-social-icons-by-guilherme-lima/bitbucket-icon

Codecademy icon
Author: -
Source: http://a714.phobos.apple.com/us/r30/Purple/v4/01/a1/3d/01a13d32-c291-b7c0-2b0d-ae1615bba381/mzl.cuoisqnr.png

GitHub icon
Author: GitHub
Source: https://github.com/logos

GitLab icon
Author: GitLab
Source: https://about.gitlab.com/press/press-kit/#logos
License: https://creativecommons.org/licenses/by-nc-sa/4.0/

Stack Overflow icon
Author: G. Pritiranjan Das
Source: http://www.softicons.com/social-media-icons/social-media-icons-by-g.-pritiranjan-das/stack-overflow-icon

Ubuntu icon
Author: Martz90
Source: http://www.softicons.com/application-icons/circle-icons-by-martz90/ubuntu-icon

Wikipedia icon
Author: Paul Robert Lloyd
Source: http://www.softicons.com/social-media-icons/social-media-icons-by-paul-robert-lloyd/wikipedia-icon