London Walks isn't free, it's 10 GBP per walk, except for those requiring an entrance fee, such as the Tower of London or Westminster Abbey. However, they're an incredible company and I strongly recommend them. Their tours are so good, it is almost worthwhile just to turn up and enjoy the walk, regardless of the topic. If you're going to do more than one, ask for the discount card. It costs 2 GBP and it will give you a 2 GBP discount on each successive walk.
From the top of my head I've done these walks.
Beatles London
Rock and Roll London
London Rock and Roll pubs
Soho walking tour
Historic Westminster
Pubs along the Thames
Tower of London
Haunted London
I'm a solo traveler, occasionally meeting up with friends along the way. The walks give me temporary companionship as well as an interesting glimpse into London. They don't always stick to the topic, either. I remember that the Rock and Roll pubs tour included a stop at a house where Mozart lived when visiting London. That was also my most memorable London Walk as the guide brought along a guitar and played some classic rock music for our group at one of the pubs.