If we are going to be talking about gear, these deserve a mention. If you have any type of artificial lighting, be it an external flash or a studio flash system, you will absolutely need a pair of Pocket Wizards. These connect to your camera hot shoe, as well as your flash and allow for off camera lighting.
If you are shooting with a Canon, you will need either the 580 vII or a hot shoe PC adapter for the 580 v1. These Transceivers (both transmitter and receiver) will help you get your flash off your camera and allow you to get super creative with your lighting. They are ABSOLUTELY worth the investment and are so much fun to play around with. You can also purchase a few accessory type cables that will allow you to trigger many different devices, including your camera. (Meaning you can hide your camera somewhere during a ceremony and trigger it by remote).
Pocket Wizard Transceiver
Is there any other gear you guys would like to know about?