In a restaurant, if the service good, 20%. If the service is lousy, 10%-15%. If the service is really friggin' lousy, perhaps no tip or round up to the next dollar, like Dr. D does.
I usually tip my barber 2 or 3 or 4 dollars, depending on how many ones I have on me.
Pizza and Chinese take-out delivery guys get 3-5 bucks (delivery is free from my favorite places here).
SiC? That you?