Off the top of my head (exluding the obvious Adolph Sax):

Gerardus Mercator (geographer/mapmaker)
Jacques Brel (singer)
Eddy Merckx (cyclist)
Kim Clijsters (tennis player)
Justine Henin-Hardenne (tennis player)
Emile Puttemans (long-distance runner)
Georges Simenon (novelist)
Andreas Vesalius (anatomist)
Rene Magritte (painter)
Peter Paul Rubens (painter)
Peter Breugel the Elder (painter)
Hieronymous Bosch (painter)
Victor Horta (architect)
Audrey Hepburn (actress)
Father Damien (leper missionary in Hawaii)

Oh dear that's 15 already and I've left so many out! Not bad for such a small country!

Peace

moo