Spotto is a music service extension for Google Chrome that provides an easy way to add songs you like on YouTube to your Spotify playlist.  

The way this works is simple: when you first install the app, it will ask you to log into your Spotto account. After this, all your playlists will appear. If you want to add a new song to a playlist, just click on the playlist you want to add the song to and you're done! You will not need to go to the Spotify website. 

Spotto automatically knows which songs are currently playing on YouTube, and knows what the name of the playlist is so you won't need to look that up.