free walking tours

I was recently in Vienna and highly recommend using the Vienna Greeters service. This is part of Global Greeters, and they ask that you donate to the organization as opposed to paying for the tour. My greeter was knowledgeable, personable, and friendly. The tour lasted 4 hours! I was expecting about 2. It is one-on-one. I have done "greeter" tours in Lyon, Vienna, and London, and highly recommend them. They tend to be very personalized, as the greeter brings their own life experience to the tour (as opposed to having a script). You really feel like a friend is showing you around "their" city.

I have done a free tour several years ago and they are ok but you will be in a large group and they do expect a tip. Personally I found it tediously slow and certainly felt that it was very superficial.
thee are two small group tours I can recommend.
https://www.nakedtourguideprague.com/
great small group tours that takes you to a variety of interesting place, their early morning tour was excellent, Marcus that runs the tours often comes on this forum with advice, nice guy and very informative
https://livingprague.com/tours/living-prague-tours/
another great small group tour max 6 people and takes you off the beaten path and down lanes an alleys that you would never find on your own.
Run by Jason who often posts on this forum.I have known Jason for many years long before he became a full time tour guide, he knows his stuff and makes tours fun and interesting.

Well I'm biased in favour of the "small group 6-8 etc" style but the question was about free tours. Certainly you'll be a little shocked if you just turn up and try to join a free tour here. There's definitely been a move to requiring you to book in advance a free tour place and you'll get a number (like Sandemans online or Discover Prague in the city). When you turn up to the tour you'll be divided into a group based on that number. Group size for English speaking free tours is around 20 and it will be suggested that you tip your guide @CZK200-400 for a 4 hour tour. If you do a free tour then be sure to ask how much of your "tip" he/she actually gets to keep. If you are somebody that wants the "sharing of personal experiences" then best stay with a private tour or small group where you'll have plenty of time to ask questions.

I would probably rent a guide. The Free Tours in Prague have usually the guides who are not locals and they do not have the licence - so they make the tours funny but with a lot of information which are not true. We rented the local guide Marketa (Prague Cool Guide) and in our next trip we did the tour with Lea - as our guide was not available and as we needed to have the scooter tour in Prague, we got the recommendation from our guide Marketa. Both young ladies were knowledgeable, helpful and the time with both of them was running really fast. We loved the tours with them and we're sure that they as the locals told us how it really is/was in the Czech Republic and Prague.

The way that I would look at this is:

You work long and hard and you save up your dollars and your vacation days for a trip to Europe. Your time in each city will be limited and will be determined to make the best out of it once you're here. A small group tour will usually cost somewhere in the region of 25 - 40 USD per person, and you're guaranteed a small group and a good (usually great) guide. Contrast that with a so-called free tour, where you'll be in a huge group of thirty people, a loud guide (they are always shouty) and you'll be expected to tip a minimum of 10 USD per person. Now ask yourself, having paid hundreds of dollars for flights and accommodation, am I really going to skimp on a few bucks now that I'm here, and end up throwing away my limited time on a tour that was a bit mmweh?

We did the Sandemans free tour in Prague. The guide was very knowledgeable and it was delivered in an interesting way. A point to note is the guides only get their money from tips, so unless it was really bad you should pay them something.
Another good tour was the Night Watchman. A guy dressed in a historical outfit takes you on a spooky tour of the old town. He does it in character, as if from hundreds of years ago. Some of the stories are a little grizzly, but its fun for kids.

I took a great free tour in Prague from GoodTours and it left from our hotel lobby. Even though I'd been to Prague before, the tour was very informative and entertaining and I learned so much more! It ended up costing me about 10 Euros with my tip for the tour guide. My friend took my advice this past fall and had the same great experience with the same company.