You can get Garen + his Dreadnaught skin for free by following Riot on Twitter

I knew about free Tristana facebook and Alistar youtube but just noticed about Garen twitter

http://2.bp.blogspot.com/-1nnoNz0r9q8/U3fjyjpf0AI/AAAAAAAACJc/L5r0dIHD3UE/s1600/League+of+Legends+Free+Skin+&+Champion+Dreadknight+Draven.jpg

https://twitter.com/leagueoflegends