Pocket-Lint: 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.