|The Noun Project|
The noun project's tag line is "Creating, Sharing and Celebrating the World's Visual Language." It started out as a kickstarter project and has become a sustaining project. here is a link to the blog post on the lessons learnt from the founders.
Contributors upload thousands of symbols for you to use. I found the search to be intuitive and very good. You can buy the icons and use them for a project. You can download and use the icons for free by following proper attribution. Also many of the icons / symbols are public domain which you can download and use without worrying about attribution.
Try it out ...