The Pillar of Autumn (Halo CE)
HaloRuns record page: http://www.haloruns.com/records?lb=101 (click the timestamps for video links)
Cryo Room/Tutorial (Easy/Normal Only)
There are two small speedups to do during the training segment at the beginning of the level.
Performing the "look at the lights" test while using inverted controls is faster than doing it with uninverted controls; the latter will force you to re-do the test using inverted controls which costs time. If you are an uninverted player, it is still faster to do the test as inverted, then quickly go into the menu after completing it to change your controls back to uninverted. If you are on pc, the inputs for this are: ESC,UP,UP,ENTER,DOWN,ENTER,DOWN,DOWN,RIGHT,DOWN,RIGHT,ENTER,UP,UP,ENTER...ESC. Practice this in the cyro tube or on any mission, as messing it up can lose a lot of time.
After completing the shield test, the crewman next to you starts a conversation with Sam, the crewman in the room overlooking the cryo area. If you get close to Sam and look at him, he will turn to look at you and will end up skipping a few seconds of dialogue. You can look at him from the ground or from the upper grate platform.
It is possible to trigger the bridge cutscene early by entering the bridge area and then backing out. The exact threshold you need to cross is before the first left turn after passing the blue "Bridge" label on the ground. An example can be seen at 3:20 into this video: 
Leaving the bridge after the cutscene as quickly as possible (moving immediately, not bumping into any walls or crewmen) will skip the trigger for the first wave of grunts outside the bridge. This also prevents you from getting a pistol, but it's still a good timesave even on legendary due to the cafeteria door opening immediately instead of requiring you to kill the grunt wave.
In the room with the U-shaped staircase, it's possible to jump from the second half of the stairs directly up to the ledge above, saving a few seconds of running. It is also possible to grenade jump grenade jump from the landing halfway up the stairs to the ledge above, but is dangerous enough that it is generally reserved for Individual Level runs or segmented runs.
Maintenance Door Skip
The first maintenance door you encounter is ordinarily locked for ~10 seconds while Cortana finishes some dialogue, but some checkpoint manipulation can allow you to skip some of that dialogue, opening the door early. Once you reach the hall with the grunts at the end, look left to trigger Cortana's dialogue, then kill the grunts, finishing the last one as Cortana says "Warning" at the beginning of her line, "Warning blast doors closing!" If you get the checkpoint as she is saying her long dialogue("We have to use the maintenance access ways...") If on 30fps, you will always get the checkpoint if you did it right, but you will only get it sometimes on 60fps. video tutorial:  example of the old way can be seen here: 
Level End Trigger
Kill the enemies, using the frags given, and a plasma or two on legendary. The level ends when every enemy in the final area is dead, and the player enters the last open escape pod bay.