(Doom Builder 2) Monster Death Events Tutorial
This tutorial teaches how to create events that happen when a particular monster(s) dies by using ZDoom ACS scripts that are called upon the death of that monster. The configuration you'll need to use is "ZDoom (Doom in Hexen format)" rather than selecting Doom or Doom 2 from the list of configurations when you first create a new map.
You can do almost anything through scripting when a monster dies, ranging from opening a door to spawning a key to teleporting you to a different part of the map and even changing the music! Your imagination is the only limit. I would also highly recommend visiting http://zdoom.org/wiki for more information on ZDoom scripting.
(Update 1/25/12)
Since it's been requested several times, here are two examples of how to make doors open when all monsters are killed. These two maps are designed exactly the same, but the scripts are slightly different. One uses a "loop" that continuously runs until all monsters are killed, then the door opens. The second method uses a "counter" which increases by one every time a monster dies. When the counter reaches a certain number, the door opens.
http://www.mediafire.com/file/02exhjo9wivb4ft/MonsterClearance_1.wad
http://www.mediafire.com/file/12za780sysibtkp/MonsterClearance_2.wad