Windows/Mac: Creating a party playlist can be daunting. If you don't trust your ability to do it yourself, Festify lets you build a playlist and then have your friends vote up songs to determine what plays next.
Once you build your playlist with Festify, you invite your friends to the list and then they can vote on tracks from their mobile devices. Whatever track has the most votes gets played next. You'll need a Spotify Premium account to use Festify.