One of the most popular icon sets is Glyphicons. Primarily made for Apple Products and iPhone Applications, they can be used in many more places like WebApps. Actually Glyphicons are mostly famous from Twitter Bootstrap.
GNOME has also its very own monochromatic icon set, it looks cool and you can use it use to any Application you want, free of charge!
Licenses & Pricing
Glyphicons come with Creative Commons Attribution 3.0 Unported (CC BY 3.0) where GNOME Symbolic Icons come with Creative Commons Attribution-Share Alike 3.0. I don’t know the differences, but in both you required to include an attribution to their pages. glyphicons.com for Glyphicons and gnome.org for GNOME Symbolic, or just refer the GNOME Project. Reading the licenses a bit, in GNOME case things is a bit more “free”.
Glyphicons with vectors (sources) cost $59 while GNOME symbolic are free (as beer).
Glyphicons are more completed for general use, but GNOME Symbolic also don’t lack much behind, and constantly are getting improved. They don’t include socials but they do include some nice emotes!
Joern Konopka ”draw” the GNOME logo with Symbolics.
This is not the completed set. To get it:
$ git clone git://git.gnome.org/gnome-icon-theme-symbolic
To get the full set of Glyphicons just visit their page, glyphicons.com.
It wouldn’t be a bad idea if GNOME Artwork could go further from GNOME borders, and make GNOME more familiar to people. Twitter Bootstrap gives this attribution:
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.
If we could change it:
To make my point clear ;)
Also there are Font Awesome Icons, which is a Icon set similar to Glyphicons, but attribution is not required.