I think copperbadge linked to the first and the second part of the evil cow-maze game.
But that's ok. After all, they only have 42 levels each.
Then I found out you can download over 1000 user-coded levels for free at the community.
No, I didn't download a pack of 125 as the perfect distraction while revising, why do you ask?
It's as bad as Lemmings. Help!