Wired has a nice article about Halo 3 focusing on the testing regime being used. Of the tools used in testing, I like the look of this time stamp map which is explained in the article:
In early tests, players wandered lost around the Jungle level: Colored dots showing player location at five-second intervals (each color is a new time stamp) were scattered randomly. So Bungie fixed the terrain to keep players from backtracking. Sure enough, the dots clustered by color, showing that players were moving smoothly through the map.
As well as being a great way to debug levels it acts as an interesting visual representation of the linearity of the level. In the above, there is basically one way through the terrain.