In Stunts, there was this great bug that made your car jump really, really high (extremely high, but you would still fall down after going up for like, 30 seconds or so). You could make custom tracks with a sequence of ramps that made this bug reproducible with high probability.
In Half Life 2 if you pick up a palet at the right angle, run onto it, then repeatedly jump, you fly. Its hardly elegant, but very interesting to see what Valve put beyond the edge of their maps and behind previously impassible police blockades.