Expedition to the Sky Citadel is the latest expansion to Card Hunter, which recently came out on Steam. It brings with it a host of new adventures, cards, and monsters for player parties to tackle. Having played it myself, the game's nothing short of a blast.
Among the new features added in the expansion are high tech cards, which lift the game from its medieval and fantasy-inspired roots to a sci-fi setting. Players can put up Force Fields to defend themselves for three turns or throw Radiation Bombs to cover foes in a random assortment of debuffs.
The new adventures in the game are designed for parties that are level 19 and above. If you haven't leveled up your party to that point, you can always use the supplied free high-level starter party to check out the new content.
Finally, co-op mode has been added to the game. Players can now take on adventures in teams of up to 3 players to win renown together.
The game was previously only available as a web-based game without a client of its own, but that has since changed with its release on Steam for the PC and Mac OSX. It's free to play, so check it out.