For portraits, you could check out Nick Knight.
For other kinds of photography, these are some of the photographers I really like.
Jay Maisel (street, former advertising photographer).
Chris Weeks (mainly street).
I would second Chase Jarvis for advertising photography, he's pretty interesting.
Ming Thein (Malaysian pro photographer, just a great shooter all round and annoyingly talented considering how young he is)
William Albert Allard
Nobuyoshi Araki (seriously NSFW)
If you're into portraits, the most famous portrait of the 20th century is probably "Afghan Girl" by Steve McCurry, and he's a great photographer too.
Nick Brandt (wildlife in Africa: absolutely incredible images of elephants, giraffes, etc. You HAVE to see his stuff).
Henri Cartier-Bresson (street, documentary). The father of modern photojournalism. Some consider him the greatest photographer of the last hundred years or so, and this is hard to argue with.
Robert Doisneau (street) Photographed Paris for most of his life. Wonderful photographer.
Chema Madoz: a surrealist photographer from Spain whose photos are incredibly creative and often very funny.
Fan Ho: a Chinese photographer with some fantastic images.
I could go on, but that should give you something to chew on!