There are four plinths to be found around Nelson’s Column, in Trafalgar Square. If you look up, you’ll see three of them have statues on them (of King George IV and two generals if you’re interested), while the fourth was never completed. Having stood completely empty for 150 years, it recently became a place of art – commissioned, of course.
Antony Gormley’s One & Other is probably the one you’ll remember (where random members of the public spent an hour each standing on the plinth). Who knows what you’ll find there today.