Building Symbols, Hospital Symbols, Railway Symbols, etc
http://dsource.in/course/design-signage/graphic-symbols
These are free to use symbols designed for use in the different environments. These are available courtesy of this website.

Airport Symbols
http://www.aiga.org/content.cfm?ContentAlias=symbolsigns
These free to use symbols designed for use in the Airport environment, that are available courtesy American Institute for Graphic Arts (AIGA).

Graphic Symbols
http://www.symbols.com/
This contains more than 2,500 Western signs, arranged into 54 groups according to their graphic characteristics. In 1,600 articles their histories, uses, and meanings are thoroughly discussed

Traffic Signs
http://www.trafficlinq.com/
Links to traffic sign and symbols

Manual of Traffic Signs
http://members.aol.com/rcmoeur/signman.html
This is a listing of the most commonly used traffic signs in the United States.

Peace Symbols
http://www.abcme.com/peacesigncollection/peacesigncoll.html
Collection of peace signs from all over the world

Funny Signs
http://www.funnysign.com/
A collection of funny signs

Symbol Signs for HCI
http://www.indiana.edu/~iirg/REFERENCES/iconbib.html
Related to Symbol/Signs for Human-Computer Interaction

International Signs Online
http://www.intlsigns.com/world/
Links to the art, law, history and humor of signs around the world