Viviane Maier was a nanny, who was very private and who is possibly one of the most important street photographers of the 20th century.

From her website:

The story of this nanny who has now wowed the world with her photography, and who incidentally recorded some of the most interesting marvels and peculiarities of Urban America in the second half of the twentieth century is seemingly beyond belief.

An American of French and Austro-Hungarian extraction, Vivian bounced between Europe and the United States before coming back to New York City in 1951. Having picked up photography just two years earlier, she would comb the streets of the Big Apple refining her artistic craft. By 1956 Vivian left the East Coast for Chicago, where she'd spend most of the rest of her life working as a caregiver. In her leisure Vivian would shoot photos that she zealously hid from the eyes of others. Taking snapshots into the late 1990′s, Maier would leave behind a body of work comprising over 100,000 negatives. Additionally Vivian's passion for documenting extended to a series of homemade documentary films and audio recordings. Interesting bits of Americana, the demolition of historic landmarks for new development, the unseen lives of ethnics and the destitute, as well as some of Chicago's most cherished sites were all meticulously catalogued by Vivian Maier.

His search for her is the subject of a documentary, which seems well worth checking out.

Official Link (WITH LOTS MORE STUFF HERE)

Documentary Website

Also,her work was previously discussed by Jenna Sauers in 2011, but I think before the creation of the documentary and I didn't know about her and think this is superneat. Finding great art outside the mainstream is always such a wonderful discovery.