All the major museums here in London are free to the public (except for special exhibitions, in particular wings, which cost). It's marvelous to be able to wander in and out, and back in again, museum to museum. So far, my favorite has been the National Portrait Gallery. But we had a lot of fun in the Victoria and Albert Museum (the stained glass, up close, was marvelous; so were the miniature portraits), the Natural History Museum (who knew dinosaur bones could be so fascinating?), the Photographer's Gallery, and the National Museum. And, as I've mentioned before, the people watching is about as good as the museum exhibits.
Here are a couple images here shot outside the front doors of the National Gallery.