Chase scenes. Best mechanic for modern action Ive seen in a long time. Every good espionage/action movie has a chase scene in it.
The chase mechanic from Spycraft was pretty good, but I wish there was a way to make it more structured. The old way was very free form where the scenery didnt factor in as much was nice, but I would enjoy random elements coming into play more that were detriments to both participants rather than just a result of a roll. For instance, the GM could lay out 'legs' of the race where certain aspects would be in play like how rough the terrain was, how crowded it was, perhaps random rolls for unusual circumstance (like a stray baby carriage or semi pulling out suddenly). You could scale it based on what kind of chase it was too. Foot chases, Car chases, etc.