It's certainly customary to tip housekeeping - as Andy says, $1 per person per night is oft cited, but it depends on the cost of the room to some extent, and those in housekeeping positions in more upscale hotels can probably expect to receive more.
Where tips are automatically added in restaurants, 18% now seems to be the norm. We usually leave 20% (who can be bothered to work out 18%?

). As the DIS article mentions, the tip is for the service, not the food. If you don't like your meal, but the service was good, you should still leave a tip as normal. We were with a friend one time who was served a really dreadful steak (no-one's fault as there was no way to know until he started eating it). The server graciously took it away and replaced it. Our friend wanted to leave a smaller tip, and he took a lot of convincing that, if anything, he should be leaving a larger tip. The same applies if you share an entree. The server should really be tipped on the number of people in your party, as if you'd all ordered entrees. I suspect this often gets overlooked.
For drinks in a bar, $1 per drink seems to be the accepted amount. If you run a tab, though, just add a tip at the end in the same way you would at a restaurant (so, for us, that's 20%).
If you order room-service, a similar percentage is expected, on top of any delivery charge levied by the hotel (as that goes to the hotel, not the employee).
We also tip taxi drivers a similar amount, although it does depend on the fare. For example, we took quite a few taxis around WDW last October and the fares tended to be in the region of $12. In that instance, we'd pay $15.
Tipping can get expensive, especially if you check in and out of multiple hotels during a trip. You are expected to tip the bellhop every time your bags are touched (and they have it down to a fine art, making sure as many of them as possible do just that). So, you tip the guy who takes them from the trunk of your car (not forgetting to also tip the valet - invariably a different guy - who parks your car). This first guy will either store your bags if you are arriving before your room is ready, or will leave them on a trolley ready for the next guy who will escort you to your room after check-in. He will tell you all sorts of things you don't need to know, both on the walk to your room and, when you arrive at your room, about the room itself; all the while smiling through gritted teeth

. If your luggage has been stored, rarely will it be delivered to your room whilst you are away - how would the bellhop get his tip? $1 a bag is no longer really enough. I see on the DIS information page, $3 a bag is suggested. That seems a lot to me, but I would say the bellhop expects at least $10 these days. Of course, you then have to go throught the whole handing money over thing when checking out. I really don't mind tipping at all and I accept that it's part of the culture in the US. I
do have a problem with this particular practice which I see as a ruse to get more money. Just call me a cynic.
