I'm really feeling this emerald trend—the jewel tone is especially beautiful for winter. Here are two nice options. I love the Piggy Polish shade—it has a little more shimmer to it. The price is a bit higher ($7.50 at Ulta.)
Sally Hansen Insta-Dri in Mint Sprint is a great value (just $3.59 at Target). Of the two it has a more glossy, satin finish. I love this product because it applies evenly and the opaque shades require only one coat.