I've been looking at using Twitter Bootstrap as a starting point for my next project and I found they were using icons from glyphicons.com. They are a set of icons that are free that require attribution and two payment options that don't require attribution.

I'm not a big fan of the attribution on the footer but for $25 it's not that bad of a deal if you need a couple of them. Making one icon usually eats at least that in my hourly fees. :-)