Walking tours in which you just "show up" like London Walks

Well, you've not got Paris on your list but there is the EXCELLENT Paris Walks company. www.paris-walks.com

These type of guided walking tours where you just show on the spot at the designated time exist in Vienna, most of them in German and in English, a few only in German. The walking tour I took a couple of years ago cost 16 Euro, you pay (cash, obviously) before it starts. The scope of the tour covered Nazi sites within walking distance from the meeting spot at Schwedenplatz given in German, some pretty esoteric stuff.

I took two free walking tours in Krakow and one in Warsaw. They were all excellent. I had the distinct impression that the tour route was set, but each guide gave a very individual commentary and none sounded scripted except for the one-sentence obligatory plug for the company's other tours.

In Dublin I booked the evening Literary Pub Crawl, it often (usually? always?) sells out and it is excellent.

Most larger cities in Europe have them, just google the city name and 'walking tours' and see what's on offer. Some of the more popular ones that have a maximum number need you to sign up ahead, most you just show up and pay the fee. I'm wary of the 'free' walking tours that, as you say, beg for tips at the end of the tour, these are usually on offer by someone other than the city. Many cities do however have 'free' walking tours that do not beg for tips at the end and, in fact, do not accept tips but it is nice if you're happy with the tour to give them something. City Greeters in several large cities are like this, most of the tours are one on one and personally designed to your interests. Research is the key.

EDIT: the one city where I took two 'free' tours that had excellent guides and were worth a generous tip at the end was in Budapest, so don't hesitate to use them there. They have several 'subject' tours - like Jewish Budapest that I found very well put together.

Also not on your list of cities, but in Frankfurt, 'Frankfurt on Foot' is excellent. Additionally they do private tours and layover tours. I highly recommend them.

Every medium to large city in Europe has walk-up tours at a decent price. I avoid the free tours like the pest, as my experience on 2 different ones have been absolutely awful and felt like a huge waste of my time. If you are going to tip 10-15€ for a tour, why not just go on a high quality tour for the same price?

Tours are listed on Trip Advisor or Google. Read the reviews, visit their website.
Don't use Viator as they are just a re-seller of tours and theses small companies have to pay a 25% commission to Viator. Don't book using Trip Advisor either as they own Viator. Just show up for the tours.

hiring a guide for your trip is a good choice. I also recommended that.

I also love London Walks, John! We did 2 walking tours with them on our last trip to London (Jack the Ripper; Sherlock Holmes) and both were excellent. We are returning to London next month and plan on 2 more - - a ghost walk and a Kensington Village walk.

We also did walking tours in Bath and in York that were super informative and really gave you a nice overview of the city. We did a rickshaw tour in Nice on our arrival day which, again, gave a fantastic overview of the city. Didn't try any in Paris. So I'm not helpful there but it seems you've gotten some great responses!