In New York City: 8.25% generally:
The tax DOES apply to prepared foods, restaurant meals, and non-food grocery items -- tp, soap, etc. -- and things like candy, soda, beer when purchased in a grocery. (Maybe it's built into the price at other sales locations, the way the tax is built into the cost of a hot-dog at Madison Square Garden.) It also does apply to clothing, but not if the total cost is under $100.
It DOES NOT apply to prescription drugs and "real" food items (as opposed to junk- or sin).
There's a different rate on personal services, like haircuts. Something lower, I think, but I've never actually checked.
The tax rate on parking lots and garages is something truly horrible like 14% or more. Maybe the idea was to discourage car use, but it doesn't seem to work, alas.
Some prices of other stuff have the tax built in, like theater/movies/sports tickets. It's all very complicated, but then, this IS NYC, after all (where life is NEVER simple!)