http://www.kongregate.com/games/fighter106/spectromancer-truth-beautyIf you're into CCG (collectable card games) mechanics or you just like Magic The Gathering, give Spectromancer a try.
You can buy it on Steam or just play it for free from the link above.
There are some limitations but you can still have a good time in the Single Duel modality, setting the enemy difficulty to "Archmage" and his deck to "Random". You can only pick the two worst decks though (Illusionist and Mechanic).
I am having a lot of fun with it, and the AI is not bad.
Rules: every turn all your powers grow by 1, and you have the possibility to play a card in one of the six slots. Then all your creatures (except the ones summoned this turn) will attack what's in the opposite slot (or the opponent if the slot is empty) subtracting their strenght to the enemy/opponent hps.
Fire, Water, Air and Earth are always there with little variation, while the fifth column changes according to the deck you are playing.
In the complete version you'll be able to play all the other decks and modalities (campaign, draft, pvp..).
Here's the card list for each deck:
http://www.spectromancer.com/cardlist_en.htm