The White Balance Lens Cap is a replacement for your standard plastic lens cap with the added benefit of helping you easily set a custom white balance.
The center of the cap has a white plastic dome like you find on light meters. Before you start photographing, you set a custom white balance from within the settings menu of your camera. The camera sets the white balance based on the reading through the white light diffusion dome and bam! Perfect custom white balance for your set of shots. Prices range from $45-65. If it’s after the fact and you need to do some white balance correcting, check out Fix Your Photos’ White Balance.