I tip in resturants about 15% for decent service, exceptional service (flirting helps!) gets 25%, and crap service gets %0 or a penny, which is what I was taught in home ec class in high school.
I tip outside baggage handlers at airports a buck a bag. (I always check bags outside, even if I walk up to the curb and use the kiosk for check-in). At Christmas, I tip the guys driving the little vans around the parking lots.
The pizza man, a couple of bucks.
I tip maids $5-10. More at Christmas. Same for my hair stylist.
I try to tip service people, like movers, garbage men (home and office), paperboys, handymen, around Christmas.
I tip the bartenders where I’m a “regular” at least $20 whenever I’m there. I’ll tip more, if I’m paying, to the waiters my roommate antagonizes.
Most of this was behavior I was carefully instructed in by my parents, a legacy of being a white, middle-class, generous Southerner. I’m grateful for the service I get, after all, and I know the folks I’m tipping need the money far more than I do. Guilt, perhaps? My hairstylist back home is a member of the church with a harder life than my family has, and slipping a couple more bucks her way seems a way of taking care of the community.
I don’t buy “college student” as an excuse, because being able to pay my first tips on my first meals was a rite of passage for me, and for my college years I overtipped because of the gleeful rush of power it brought. This has been weened out of me by exceptionally bad service, so my range of behavior has become greater.
As for tipping in groups, this was definately a problem in college when we were all broke and starving, but now that I’m an adult the people I go out with generally expect to pay their share of the ticket AND the tip.