Iconfinder is an icon search engine that fetches relevant icons depending on what type, color or size you're looking for. As they have around 70 thousand icons and it's overall nice and very simple to use I would definately say it will come in handy!
http://www.iconfinder.net/
I was very fortunate to
hear Jared Spool speak at last years SXSW festival.
This article documents how a small change to a usability workflow increased an online retailers sales by $300 million.