I've always admired the work of great street photographers like Gary Winogrand.
This is a photo I happened to take myself a few months ago. To see quite a few more (by other people) just search Flickr for "street photography".
Looking online, I've found endless discussion about the techniques and ethics of street photography. This discussion thread on Photo.net is interesting. And this PDF book by Chris Weeks, Street Photography for the Purist is quite rich, with illustrated intros by several other street photographers. I found both these links, by the way, in the Wikipedia article on Street Photography.