Iconathon.

“The Iconathon is an initiative to collaboratively design new civic symbols for the public domain.… In August & September 2011, several cities across the US will participate in a series of design charrettes — day-long collaborative workshops — called ‘Iconathons’. Through facilitated design sessions, event participants will generate icons and symbols that visually convey concepts frequently needed in civic design.”

The events are co-organized by The Noun Project which hosts a large number of public domain and creative-commons license icons you can download.