I've just posted a tread about the Ansel Adams show at the National Maritime Museum, (brilliant!) but I've been in London the last 3 days, and there are some other good Photo exhibitions on well worth seeing if you're in the capital. The Klein/Moriyama show of course, which is the best thing I've seen at The Tate Modern.
The V&A has some interesting work by Middle Eastern photographers, and its free, as is the Cartier-Bresson influenced exhibition at Somerset House.