It's hard to pinpoint exactly when it started (Chanel's sold out black polish perhaps? All the limited edition OPI collections?), but nail polish trends are as feverish as fashion ones. Better yet, while you may not be able to afford a new designer wardrobe every season, most of us can afford to revamp our nail polish collection every few months. To that end, here's a look at the latest polishes worth investing in, but hurry up…many of them are limited edition.