What are some good exhibitions on at the moment (art / education / whatever)? Or just good museums generally (in addition to the major V&A, Natural History, British, etc).

(I have a day of annual leave I have to use tomorrow, and normally I use all my annual leave to go on holiday i.e. out of London/UK, but I have one day leftover and want to use it productively. I'll /probably/ go to a couple of free things as I've just spent on PJ Harvey tickets for tomorrow night after realising some were still available. Sweet).