I'd say Postal 2 is downright mental, or the designers at least, has to be... Not neccessarily a bad thing, but mental all the same
Monty Python grants some very strange games indeed. Being chased by huge feet is not something I experience on a daily basis.
And, on an old all time favourite of mine: The concept of Lazy Jones is kinda weird: A game about playing computer games.
EDIT: A concept that I found intriguing, was to have two games compatible against each other, like Apache and Hind, only it would be fun in a deranged way to have Apache VS say, Formula 1. I wonder what it would be like to come down the stretch on Hockenheim, driving like a maniac to escape that Hellfire missile at your 6 o'clock.