I have known many people that work in the following situations and can tell you that the following guidelines are all that they are expecting. But if they can get more, they'll take it!!!! It isn't considered "necessary" to tip the person who gets your bag out of the car, but it's certainly at your discretion if you would like to.
Valet Parking - $1 to $2 per car is usually appropriate, given when the car is returned to you. However, if you think that you may need to leave in a rush, you should also give the attendant a tip on the front end and request that your car be placed somewhere that it can be retrieved quickly.
Taxi 10% to 15% of the total fare.
Limos 15% to 20% of the total bill.
Maid - $1 - $10 per night, depending on how expensive the room is, and how messy you are.
Room Service 10% to 15% of total bill. Many hotels add the gratuity to your bill automatically, so be sure to check your bill when the food is delivered.
Bell Desk - $2 - $3 per bag for delivering bags to your room; $1 - $2 per bag for retrieving your bags from storage.