In addition to many of the well-known photographers already mentioned, I've learned of a relatively unknown photographer whose street shots are really very good. Her name is Vivian Maier, and her work can be found on the blog
Vivian Maier - Her Discovered Work. Here's John Maloof's, the blog owner, forward:
"This was created in dedication to the photographer Vivian Maier, a street photographer from the 1950s - 1990s. Vivian's work was discovered at an auction here in Chicago where she resided most of her life. Her discovered work includes about 100,000 mostly medium format negatives and a ton of undeveloped rolls of film. Born February 1, 1926 and deceased on Tuesday, April 21, 2009."
It's worth a look, if for no other reason than the glimpse of life during the period Maier was shooting in Chicago, though what's known of her life story is fascinating.