[1500 vector symbols in 32 libraries, free for any use, from the Integration and Application Network of the University of Maryland Center for Environmental Science.][link]
The icons are of varying quality, but it’s certainly a useful round to have in the chamber. [Via Zeldman][zeld].
[link]: http://ian.umces.edu/symbols/ “IAN Symbol Libraries (Free Vector Symbols for Illustrator) – Integration and Application Network”
[zeld]: http://www.zeldman.com/2010/12/16/science-illustrations-cost-and-royalty-free/ “SCIENCE ILLUSTRATIONS, COST AND ROYALTY FREE”