It's not a conventional visitor attraction but only the terminally uninquisitive would pass up an invitation to look inside this extraordinary helical steel-and-glass building, designed by Foster & Partners, on the South Bank of the Thames, adjacent to Tower Bridge. From one angle it looks like Darth Vader's helmet and has also (jokily) been nicknamed "The Glass Testicle". It has been the home of London's governing body, the Greater London Authority (GLA), since 2002. There's a café inside, great river views, and the intriguing London Photomat – the biggest aerial photograph ever taken of Greater London, at a detail precise enough to pinpoint individual houses and buildings. And it's all free of charge.