Inventory is an asset which allows your character to collect items throughout your game and utilise them for whatever purpose you want. Not just for platform games, with some tweaking this can fit pretty much any game.
Blog post: http://www.blurrygames.com/archives/994
Executable demo version: http://www.blurrygames.com/play/Inventory/download/Inventory.exe
This asset should be easily added to most Game Maker Studio projects. It may require just a small bit of work to get everything running smoothly, but trust me the result will be worth it.
The asset once purchased will be endlessly customisable, for example you can add your own variables to the items, your imagination will be your only limitation to what you use it for.
I HIGHLY RECOMMEND you read the guide located in the scripts section, it will give you some basic introduction to using the asset.
There are some minor collision bugs in this asset however remember this isn't a platform asset, its an inventory asset.
So far I have only tested this on a Windows 7 PC and so I am unsure of its compatibility with other platforms.