Susan Kare is undoubtedly a pioneer of user interface graphics and pixel art. She worked on the original Macintosh Team at Apple and joined Steve Jobs at NeXT as a Creative Director. Among her portfolio many famous clients can be found, e.g. Apple, Facebook, Microsoft, Paypal, Glam and many others. If you are interested in any of her artwork, limited edition prints both signed and numbered can be bought at kareprints.com
This is a talk regarding the design of icons and symbols and is actually quite interesting. So, check it out.