GLYPHICONS is a library of precisely prepared monochromatic icons and symbols, created with an emphasis on simplicity and easy orientation.

GLYPHICONS FREE are released under the Creative Commons Attribution 3.0 Unported (CC BY 3.0). The GLYPHICONS FREE can be used both commercially and for personal use, but you must always add a link to GLYPHICONS.com in a prominent place (e.g. the footer of a website), include the CC-BY license and the reference to GLYPHICONS.com on every page using icons. This is very restrictive and could make your page look ugly.

There are also commecial versions available.