Most Belgian students know about the Go-Pass, valid until 25 years old, which allows you to travel between any two stations inside Belgium for 4.5 euro per journey. The pass includes 10 journeys and can be used by more than one person at the same time (e.g. when travelling with friends).
The same pass also exist in "adult" version (from 26 years old) for 6.8 euro per journey. Ask for a Rail Pass.
What less people know is that yet another similar pass exists, which anybody can buy, and allows one to travel short-distance (usually within 20km, e.g. all the Walloon and Flemish Brabant provinces from Brussels) for only 1.5 euro per journey. This is called the Key Card.
Many other discounts are available, such as season tickets, although they are usually not so intersting except for commuters.