1.- Music Volume Handling in ACS. This is usefull when changing a song in the middle of the map, to decrease the volume until it is zero, and then start another song, so we reduce the feeling of "cutting" one song and starting another.
2.- Player_HeightActivates(PlayerID, Height, ScriptNum). This is not an ACS or linedef special. This is an actor, invisible ingame, such as a Map Spot. This actor, will check if "PlayerID" reaches a certain "Height" in the sector containing this actor. If the player reaches that "Height", then the script "ScriptNum" will be activated.
This is usefull, for example, for "realistic" elevators, where the player is suposed to enter and leave it by the same side.
If we check the player height, we can teleport him when the fake elevator lower/rises to a certain height.
As general purpose, it's very usefull if we want to build a large part of the map below another part, and when the player jumps down or climbs up from a gap or hole or similar, avoiding the mapper to use lots of 3DFloors, and making more realistic constructions.
3.- Water Shaders, to make an effect like the one saw on this image:
That image is a fake i did on Gimp, but it shows what kind of shader i'm refering to.
I'm quite interested in the second suggestion, as it will open us all lots of new possibilities.