Want to try even more for free? Try these:
Free speech is alive and kicking every Sunday at Speakers' Corner in Hyde Park on the corner of Park Lane and Cumberland Gate, nearest tube: Marble Arch. Ever since the Royal Parks and Garden's Act of 1872 permitted free assembly, speakers - and listeners - have gathered at Speakers' Corner to set the world right. Karl Marx and Lenin both used this platform to spread their beliefs but these days you're more likely to hear less worthy orators. Some speakers draw crowds and if you fancy a rant you can stand up and tell the world your views too, but expect plenty of heckling. The main topics tend to be politics and religion but there are usually some off-the-wall ideas too.
2. Free Art Tours
In the 1st Top Ten Free Things to Do in London I listed London's art galleries and museums. To get even more for nothing try some of their free tours. Most run daily and you don't need to book - just turn up and enjoy the knowledge of a specialist guide. I like the tours at Tate Modern, Tate Britain, and the British Museum. Try the ArtStart interactive screens at the National Gallery to put together your tour and print it there. If you have children, the major art galleries and museums run free family activities, especially at the weekend and during school holidays. At other times there are usually handouts with children's activities or trails available.
3. Platform for Art
Transport for London have some interesting art ideas with their Platform for Art public art program using a tube station platform as an art gallery. Go to Gloucester Road station on the District and Circle line to see this makeshift art gallery on platform 4. In 2006 there were murals from the Japanese artist Chiho Aoshima, and other shows have included a 7.5 meter wide giant panda's head and a pile of used mattresses. Yes, this is contemporary art at its best. Over 21,000 people pass through the station each day, making Gloucester Road Underground's disused platform one of the most visited art spaces in Europe. Go to the Eastbound District and Circle Line platform but also good views from the Westbound platform.
It's a classic photo opportunity: have your photo taken standing on the Prime Meridian Line at Greenwich. Head up the hill to The Royal Observatory and in the courtyard is a metal strip where you stand over the line and can be in the the eastern and western hemispheres at the same time. Greenwich was chosen back in 1884 as the Prime Meridian of the world, Longitude Zero (0° 0' 0"). Every place on Earth is measured in terms of its angle east or west from this line (longitude), just as the Equator divides the northern and southern hemispheres (latitude). Another fun freebie while you're there is to watch the red Time Ball on top of Flamsteed House (next to the Meridian Line) drop at 1pm every day.
5. British Library
The British Library is the national library of the United Kingdom and one of the world's greatest libraries. Everyone can visit the exhibition galleries or tour the building. However, to use the Reading Rooms, you need to register for a Reading Pass. There are free tours of the building and conservation work as well as regular Tuesday lunchtimes talks at 12.30-1pm. See What's On for the latest events.
6. Leicester Square Film Premieres
Leicester Square hosts 50 film premieres every year and it can be exciting to see the stars arrive. Many celebs enjoy meeting the crowds here, so get there early for a good spot at the front of the barriers and be ready to call loudly when your favourite star appears. Find out what film premieres are planned on the Westminster City Council website.
June 2012 update: I think this has now left The O2, Sorry!Head to the O2 in Greenwich where there are facilities for you to sing along to a famous song and be superimposed into the video! The video is them emailed to your mobile phone or email address for free. If you're feeling shy they have lots of props for dressing up as everyone can watch you dancing away. I think this is worth a trip to The O2 on its own!
London Zoo is in Regent's Park which means there are a few places you can see into the zoo for free. If you walk along Regent's Canal you can see some of the Into Africa animals and the bird aviaries. Cross over Primrose Hill Bridge (over Regent's Canal) to see Red River Hogs and okapis (Into Africa). And for my favourite, walk along the Regent's Park Outer Circle, past the London Zoo entrance, opposite the West Service Gate, and see the giraffes.
10. Auction House Viewings
As I did on the first Top Ten Things to Do in London, here's one more idea:
Sit between bronze statues of Churchill and Roosevelt who are sitting on a park bench where New Bond Street meets Old Bond Street. This is called "Allies" and is by Lawrence Holofcener.