There is an abundance of 18% gray gadgets to help you get your white balance spot on. The most common is a graycard, a simple 18% gray plastic card that comes in different sizes. I’ve also come across 18% gray lens cloths and and this nifty little Expodisc which looks like a filter and according to the manufacturers “simulates a perfectly-balanced gray card in almost any situation”.
Getting the white balance right can make a world of difference in a lot of cases. Most of these involve  shooting jpg, or when balancing the light temperature when working with different types of light, say strobes and static studio lights. Correct white balance straight out of the camera can streamline your workflow but it’ll provide nothing that you can’t do with your images in post. Shooting raw eliminates the problem of messing up an entire shoot because of bad white balance  and given the low price of storage these days there is really no reason why anyone serious about photography would shoot jpg unless it’s for the sheer speed of having ready images right out the camera, say news or sports shooters who dump their cards to the newsroom’s server straight after the game.
An Ipod touch or Iphone is a pretty handy thing to have because you can store your entire portfolio on it and display it to  potential customers.  It’s even more useful with the slew of photography related apps available. GrayCard by Pixelexip,  does exactly  what the name says. It allows for temperature compensation as well as providing a very intuitive GUI to work with. At 7 SEK/$0.99,  it won’t break the bank either.
Here are the screengrabs: