These nails are one of my favorites. Why? They're easy, quick, and you can use any nail polish you have! I used a couple colors from the Sally Hansen Insta-Dri collection. They're -extremely- pigmented and dry super fast. The only drawback is that when doing gradients, you must work quickly or else if you overblot it, pieces from the makeup sponge will start sticking to the nail.
If you're interested in trying this out, TheBeautyDepartment.com has a great tutorial for it. You literally wet a makeup sponge, squeeze outthe excess, apply your two colors on it (I prefer the cornering the thinner end) and press lightly. Then repeat until you reach your desired effect (3-5 times for me depending on the polish). Ta da! Have fun!