Summary
37signalsが配布しているアイコンCC0 1.0 Universal license / WTFPL。
Reference
Open Source Icons - (37signals)http://37signals.com/svn/posts/1844-open-source-icons
Creative Commons - CC0 1.0 Universal
http://creativecommons.org/publicdomain/zero/1.0
Wikipedia - WTFPL
via
tubuclip - abouthttp://tubuclip.com/about