Zoya does some version of this sale every year, and while it's a good deal, I kind of hate the way they promote it. According to them, you get 4 free polishes of your choice (some products excluded), but you have to pay "promotional shipping" of $15, which is higher than their usual shipping cost. So really you're getting 4 polishes for $3.75 each with free shipping. At least that seems to me a better way to think about it. Great deal, still--last time it was only 3 polishes for $15!
Details here. Use code 4YOU. Ends Jan. 13.
Are you going to get in on it? Zoya is probably my favorite nail polish brand. You can see some of my swatches in my Zoya tag.