Sometimes the small things are the best. CardNinja is a simple yet effective "wallet replacement" - or more of a card pocket really - that can stick on the back of most smartphones or cases.We know you're probably already thinking that your cards will fall out, but CardNinja has a sensei-like grip that can hold in one card or a whole bunch. We not only held it upside down, we positively jiggled it around to see if a lone Visa would go flying. But it didn't.The adhesive pocket won't stick on to rubber-like surfaces, so some cases and the odd phone won't be compatible, but most will. Once it's on it's solid too. We gave it a good tug to try to peel it away from the phone case, but it stayed put.We can see a lot of practical use for the CardNinja: Subway or Oyster cards needn't go missing again; bulging wallets needn't be seen protruding from back pockets; or when you've got those dancing shoes on it offers a practical way to cut down on what you're carrying.Available in black, grey or purple, the CardNinja smartphone wallet costs $20. International shipping is available direct from the CardNinja website as there's no current distribution outside of the USA.