I always tip where it's expected because I spent a large part of my youth working for tips and it really sucks when someone stiffs you.
Restaurants: At least 15%, but 20 or more for great service. If they really went out of their way, I think it has to be at least 20%, especially if you're in a group and they still were awesome. When they tack on that automatic 18%, if they were really great, I still chip in more.
Delivery guys: I've been a pizza guy so I ALWAYS tip, even if they aren't fast. If you're a regular pizza orderer from that place, it'll mean everything from your house becoming known as "one that tips" (some drivers will bring your pie before other homes sometimes) and if you're a longtime regular, sometimes it even gets you better pizza.
Cabbies: I'm not a big fan of tipping them a ton (I actually had one tell me I wasn't tipping him enough once)...but I do. The only time they get a great tip is if they drive how I ask them to (like..."go fast").
Bartenders: Of course. It's stupid not to if you want a good drink or fast service.
Drink girls in Vegas: They're bringing you a free drink, you have to.
Blackjack Dealers: Only if I'm winning though. I guess that's wrong.
Tour Guides: I climbed Mt. Rainier a few years ago and found out at the end that I was expected to tip the guy I had just paid 600 bucks to guide me up the mountain. I mailed it to him, but that one shocked me. I've decided my stance is if I got a lot of personal attention on the tour (like a van tour) I'll tip, but if I was on a bus with 40 people...nah.
Hotel maids: I dated one in high school and she told me it's really nice when you leave a tip in the room. The weird thing is when to leave it. She says daily, so I always leave some out for the maid. In Vegas, they only take it half the time (I think because everyone leaves cash in the rooms there) but in other cities, it's always taken.
Here's who I don't tip:
Skycab: I see these guys like doormen, but I recently found out you can only use them IF you tip them. Since Sept. 11th, they stopped saving me time (too much security), so I never accept their help anymore.
Doormen: I almost never tip that guy at a hotel (especially the vegas ones) that opens the cab door for you. That is a retarded expectation. Unless I'm drunk. If I actually ASK for their help (like, here...help me with my suitcase) then I'll tip them, but if they take it or something, I don't want their help and I won't pay for it.
Hotel front desk clerks: I've heard you can get a better room if you tip them. It's never worked, so I don't do it anymore.
Restaurant Hosts: Again, I'm not a high roller I guess, but tipping these guys is not something I do.
Fast food workers who put a cup on the counter. Sorry. Unless you brought me my food, you didn't perform an extra service. I'm not putting something in the cup just because you rang up my burger.
Starbucks employees: Same deal as fast food. I used to tip when I was a regular at the little mom and pop shacks, but I don't tip Starbucks counter people these days when I occasionally buy coffee. I used to, but I stopped when I stopped being a regular because I don't see the point anymore.
Bums: I know it's not tipping, but I'm not fond of giving out money to beggars or the "Will Work for food" guys. I rarely do it and when I do, it's because of a special circumstance (like a really funny sign, or a genuniely needy looking person).
Strippers: Yes, this is a funny one...but some strippers actually expect you to tip em. Not a chance. The only time I'll give them extra money is if I'm in that front row, and that's because a dollar bill is an incentive to bring them over my way.
Street Performers: Only if I stop to watch the show for a few minutes.