I don't know why I've never stumbled across symbols.com before, but it's a wonderful, unique site. I wish I'd have seen it when I was putting together my row of buttons up there, since there's a few more esoteric symbols I wanted to include. Maybe I'll get around to it now. Check it out:
SYMBOLS.com 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. The signs range from ideograms carved in mammoth teeth by Cro-Magnon men, to hobo signs and subway graffiti.
Be especially sure to check out the graphic search, a very interesting attempt at taxonomical search of visual data.
