Basically what you do is take a browse round their mashed-up Google map of the capital and choose a building to sponsor. It doesn't have to be Big Ben or the Millennium Eye, it could be your local pub, tube station or school (there's a sliding price scale obviously). Then you get to add photos and a description for whatever you've bought.
It's a great idea and the site is reallly well done...if only we could decide what to sponsor (if you've got a suggestion as to what Londonist should adopt then do let us know in the comments).
(Thanks to Londonist reader Carl for the tip off).