List of free skins in League of Legends

The list is preety short and only 3 of those  can be obtained for free at this moment.

1. UFO Corki ,beta testers got it for free
2. Judgement Kayle free ,for all that played 10+ ranked games in season 1
3. Victorious Jarvan IV ,free for all players ending with Gold or above ranking in Season 1 
4. Grey Warwick ,free for those who refer 50 friends to League of Legends
5. Unchained Alistar ,free for those who subscribe to Riot's Youtube Channel
6. Riot Girl Tristana ,free for those who join Riot's Facebook page

U can still get the Tristana and Alistar (both of them come with the free champion) skins for free quite easily,but the Warwick skin is really hard to get.