London: Free sights! from Budget Travel

This summer, we have to be prepared for Volcano eruptions and flight interruptions! Will the gods be on our side during our next trip to Europe? Hard to predict, so in case you get stuck in London and you don't want to spend more of your hard earned cash in daily activities, here's a handy list of some FREE sights in London published by Budget Travel! London: Looking for a silver lining? Free sights! - This Just In - Budget Travel

Can you think of any other cheap or free sights in London?