The Tate Modern on the South Bank opposite St. Paul's Cathedral has had a huge new part opened recently, and on the 10th floor of the new section is an open air 360deg. viewing terrace. It's free to visit and has a little cafe up there. Wonderful views, around the height of the London Eye I'm guessing.
This is the second time i've been and i had to get there at opening time (10am); last time, the queues for the only lifts that go to lvl 10 were horrendous and i gave up waiting, but it was a weekend lunchtime i.e. Prime visiting time.