Resturant	15-20%
Buffet	5-10%
Bartender	$1-2 a round -or- 15% to 20% of the tab, with a minimum of 50 cents per soft drink, $1 per alcoholic drink
Cocktail Waitresses	$1-2 per drink
Take-out Delivery	10-15%
Catering	20%
Floral deliveries	$1-3 Complicated or large plants up to $10
Hair stylist	15-20%
Hotel housekeeper	$2-5/night - left daily in a sealed envelope.
Valet	$2-5
SkyCap/BellHop	$2 first bag, $1 per additional bag
Concierge	$5 for tickets or reservations, $10 if hard to get; no need to tip for answering questions
Taxi driver	15-20%
Contractor/ tradespeople/ craftspeople	Boston, Mass., etiquette consultant Jodi R.R. Smith: Don’t tip skilled craftsmen or technical specialists, such as plumbers, electricians, painters, alarm service technicians, handymen, piano tuners, or appliance repair people. "That’s like trying to tip a doctor or a teacher," Contracto comment: "As an independent contractor, I'm not going to say no to someone handing me some extra cash but I don't expect it in the least; Across an average year, I might get handed a total of an extra $250 and I just quietly put that in my kids' piggy banks. The thing that makes me like clients the most is offers of cold water and/or hot coffee."

Links:
Tip20! - Tipping Information, advice, standards & practices - Also has International Standards
General Tipping Guidelines at EmilyPost.com