Many's the time I've been tempted to open a bottle of nail polish in the shop and give it a sneaky try out on my nails. I don't obviously, because that would be stealing, but now OPI have introduced a cute feature to their website which lets you try on their colours before you go out and buy them.
Head over to OPI, click on the Try-On This Colour tab, use the sliders to select your skin tone and nail length and test to your wee heart's content. The vagaries of computer monitors mean that the colours will never be an exact match, but it's a great way to get an idea of how a colour will look on you before you go out and spend your money only to find you hate the colour on.
The picture by the way is what Sahara Sapphire will look like on me. If my nails were longer. And my fingers were longer!