It's not exactly the most glam museum in London, but the National Portrait Gallery is still one of my favorite places to kill a couple of hours in an afternoon, especially when it's bitter cold outside and you're looking for something to do on the cheap. It's a slog through the tourist crowds to get there (the NPG is the National Gallery's caboose, right between Leicester Square and Trafalgar Square) and the gallery is smallish - you can do the whole thing in the aforementioned couple of hours if you want - but what a gallery.
It's like walking through the Head Museum in Futurama, except that the heads are paintings and they don't talk. OK, it's only remotely similar to the head museum. But for people who enjoy a good romp through history's halls, it's like being with the characters from your favorite book. Richard III? Henry VIII? Charles II? All there. The symbolism in some of the earlier paintings is like a Where's Waldo for history buffs.
The NPG is one of the little unsung heroes of my city. And I do love it.