Justin:

Big time deals.

Especially in areas served by several taxi companys. The Hotel staff get "tipped" by either the driver or the car service company.

I've found it's important to check the local taxi rates in advance, by calling the taxi company directly. Sometimes the Bell staff will arange for a private car service and I've had ones that tried to charge me far more than the standard taxi fare.

Some times I've seen taxi drivers slip a few bucks to the bell-hop.

I even reported one obvious rip-off attempt to the hotel.