Original Link: http://fortawesome.github.com/Font-Awesome/

Font Awesome is a handy iconic font designed specifically for use with Twitter Bootstrap, but can be used elsewhere as well.

Yes, Bootstrap does currently have icons built in, but they are image sprites which means you are limited to the colors they include, which are white or black..

Using an iconic font like Font Awesome, gives you more freedom when it comes to having it blend, plus it actually has more icons that bootstrap normally includes.