We have various threads about various online distribution platforms, but a lot of them have gone and dropped far into the forum depth. So as not to practice forbidden dark arts of Threadomancy:
Green Man Gaming has a Battle Trhough the Ages deal going. Mentioning it because it has all the Crusader Kings II stuff going 50% off. That is steam keys.
Personally I did not really like the game, but a lot of forum regulars here absolutley love it.
My own taste in the sale goes more towards the