There's a shuttle bus from Charleroi airport to Brussels, but if you book it a couple of days before it costs 15 euros plus 3 euros booking. A train could be a lot cheaper but it's unclear how to get to the train station from the airport.
I have the feeling hitchhiking wouldn't be so hard, but every time I was there it was raining and/or I was already too tired.
What's a good and cheap way to get between Charleroi airport and Brussels?
Note: Charleroi airport is not Charleroi.