Most of the major European cities have "free" walking tours. Format is this:
1) you meet up at a major attraction (i.e. Notre Dame)
2) you find the guide (who is usually holding up some sort of sign or wearing a bright piece of company clothing
3) you wait a bit for other people to show up...maybe 5-10 mins
4) guide introduces themselves, asks around who is from where, etc.
5) you start touring the city and walking around the attractions
6) guide is usually pretty informative and quite friendly...if you have any questions, fire them away
7) you end up at your last stop. Guide will tell you that it's free but they accept tips.
8) I usually give them about £10 or 10 euros.
9) you thank them and be on your way. I also like to ask the guides if they know any local "hole in the wall" type eateries...I find those the best and usually the most fun.
I've gone with Discover Walks. Generally a good team (I've done their Paris and Barcelona tours).
Barcelona Walking Tours – free guided tours of Barcelona
Also did New Europe tours...not as good I think but generally same format (did this for London):
«Home of the Famous London Free Tour with some of the best guides in Europe» - SANDEMANs NEW EUROPE