This is rather an example I came up with today while learning for making Platform games.
Sudgestions and examples with other ideas for this topic are welcome!
Example contains:
-Basic description in eventsheet
-Character can jump trough and land on platform object
-By pressing down the player drops down trough the platform
File: dl.dropbox.com/u/5214903/GM/JumpTroughPlatform65.zip
Sorry if there was already a solution made