I know that there are already a few platform widgets out there, but I just made one of my own, in case you are interested in seeing it.
Customize-able speed, jump height, gravity, etc.
Jump through platforms and slopes.
Uses the platform movement object, allowing for easy modification.
Can automatically handle a character object, complete with animations (basically, that's the animated object that doesn't handle the collision detection, labelled as "Blob" in the "With character" frame).
Allows you to manually handle your character object (by using the objects from the "Without character" frame).
The jumping and falling animation don't play properly (only a problem in the "With character" frame).
Jump-through platforms don't always work properly (to see what I mean, run the first frame, climb up the slope to the right, then head left until you are at the bottom of the slope).
The "Character object" isn't fully animated (only the walking, falling, and jumping animation implemented, with the falling and jumping animations not working correctly under most circumstances), meaning that you will have to manually implement any further animations.
Platform movement object.
Objects that have behaviors:
"Main" (the platform movement object).
"Jump through platform" (delete the events related to it if you don't want to use jump through platforms, or you want to implement jump-through platforms manually),
and "Blob" (if you are using the objects from the "With character" frame and you are wanting to use the automatic character positioning and automatic character animation feature).