Here's a cool little thing I picked up from Amazon last week. It's a handy little tool to have in your photo bag so I wanted to share it with you. I've been having a devil of a time getting accurate colours from my Nex-7 in the constantly changing Irish daylight, so I wanted to get a grey card so I could accurately measure the proper white balance in a given light set up. I searched around a few local camera shops and all they had were the canvas type fold out ones (that are like a small reflector) which isn't much use for walking around with. Anyway, a quick search on amazon turned up this little number and a short trip through the postal service later and it was in my possession.
It's pretty much as you see it, but it's small and very neat, and comes with a handy lanyard to hang it around your neck. The size is pretty much the same as a credit card. If you're having trouble getting the right white balance or are shooting in mixed lighting I can't recommend something like this enough. Just snap a shot of it in situ in your lighting conditions and then you can use it to white balance off of. Then you can copy and paste the settings to the rest of your images. Anyway, there's lots of different ones available but this one works well and I can heartily recommend it.