"Glyphicons Halflings are normally not available for free, but an arrangement between Bootstrap and the Glyphicons creators have made this possible at no cost to you as developers. As a thank you, we ask you to include an optional link back to Glyphicons whenever practical."
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About Glyphicons
From Bootstrap docs (http://twitter.github.io/bootstrap/base-css.html#icons):
"Glyphicons Halflings are normally not available for free, but an arrangement between Bootstrap and the Glyphicons creators have made this possible at no cost to you as developers. As a thank you, we ask you to include an optional link back to Glyphicons whenever practical."
Font Awesome's license makes it effectively free:
http://fortawesome.github.io/Font-Awesome/license/
The Font Awesome licence is much better (permissive) than the Glyphicons one.