Blogged in The Woodwork: How (I purchased a kit) dSLR [The entry kit dSLR Part 5]
White balancing cap
South of Market, San Francisco, California
Olympus E-P2, Lumix G Vario HD 1:4-0-5.8/14-140 ASPH. MEGA O.I.S.
Adobe Camera RAW, nik Sharpener Pro (raw presharpen), Topaz DeNoise, Kubota Image Tools (hawaiian punch)
1/13sec @ ƒ4.5, ISO2500, 22mm (44mm)
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The lens cap for the rear element of the 18-55 VR kit lens is more cheaply made than usual for Nikon (it is the same one supplied with the 50mm f/1.8). But it has a second, secret use. You can use it as a ambient white balancer.
I tested this using PRE white balance (you have to use the menus, not info to set) on a Nikon D5000.
Shooting notes: I actually white balanced the photo using the same trick on my E-P2. Unfortunately, the lighting was so bad, no detail is recoverable at ISO 2500.