The Binding of Isaac
Despite the screaming aborted fetuses attacking you, a bunch of weird Freudian shit and the German rating for blasphemy, the Binding of Isaac is a very funny and creative game, and the only thing really traumatizing is the difficulty (well, that, and those particular enemies that cower away from you, whimpering, as you try to kill them). I recommend it to anyone looking for something more challenging than Solitaire. It's only € 4.99 anyway: http://www.bindingofisaac.com