Some of good, some not so good.
One of the best that I have experienced in my global travels is Venicescapes (
http://venicescapes.org/guided-histo...urs-venice.htm.)
Personalized, lively, and informative tours of Venice by a very knowledgeable guide.
In India, we enjoyed several private excursions with Mumbai Magic. The Good Morning Mumbai tour through the early morning flower market (seeing the flowers arriving from the countryside) and the fish market (seeing the fish being unloaded from the boats) was excellent as were several others we took.