Heroine's Quest: The Herald of Ragnarok had Steam trading card support added on 7 April 2014. There are 8 cards in the series.
Unlike with paid games on Steam, where playing the game drops 1/2 of the cards in the set rounded up, or Free to Play games where cards drop through purchasing in-game content, for a free game there's no way to directly earn cards. It is possible, however, to craft booster packs for the game using Gems. To do so, you must first install the game in Steam to register it as being associated with your account.