Free Tours Of London offers free tours of London. I am very impressed by this service as you will not find anything for free in London. There are 3 kinds of tours:
Royal Tour
Old City Tour
Jack the Ripper Tour
You can find all the details on the website including timings and highlights of the tour under the Tours tab.
There is also a Blog section where you can find about all the events going on in London.
I am more of an ‘Old City’ person. so this summer I am definitely going to one of these tours, although I live in London but touring with other people has a different charm to it, I also need to cut down on just going to central London for shopping.