A crate for packing soap.
Any physical or media platform which gives prominence to the person on it and the views they espouse.
The madman obtained a soapbox which he stood on at the corner of Broadway and Wall street, to shout out his prophesy of the end of the world.
A talk about one's pet topic (or the topic itself), especially when only tangentially relevant to an ongoing discussion.
He's been on his soapbox all day about the new football coach.
A soapbox car.
To give a speech from (or as if from) a soapbox.