London is a notoriously expensive city.  But you can have fun there without spending any money.  Here are some activities in London that cost zero pounds, dollars, euros, pesos, yen, soles, etc.

Museum of London: Free, but the lower galleries are closed until 2010 for renovation and improvements.

the Women’s Library The largest collection of women’s history in the U.K., according to the website.  Check it out!

Fulham Palace: this estate was owned by the Bishops of London for over 1300 years.

Walk Hampstead Heath.  Why not?

Fly a kite, go rollerblading, ride a bike, or go skateboarding in one of London’s parks.  There are also often free talks and guided walks in the royal parks, so be sure to check that out at the Royal Parks website.

Attend an auction at Christie’s.

View the Changing of the Guard.  How could you go to London and not see this?

This is just a short list to get you started.  There are hundreds of things in London that don’t cost anything, so keep that in mind as you plan your itinerary.  Some museums are free, as are libraries, art galleries, street performers, street markets, Westminster Abbey, Trafalgar Square, and, of course, just walking around (the South Bank, say.  You can download free walking tours at Walk This Way.)