I'll give the shuttle driver $2 if (s)he helps me with my bags.

Taxi, around 15%.

I don't tend to "use" the services of the doorman, and I carry my own bags to my room, so no tipping there.

Room service in most places already includes a 15 to 18% service charge. I don't tip on top of that. Be aware that there is often a tip line on the charge slip-don't let that fool you, check to see if there was a fee already.

I do leave the maid a few dollars as well.