Besides potentially being the "rich owner" of the Riddle House and giving suspicious donations to St. Mungo's (because, you know, he *really* cares about the well being of innocent people
), I don't know. I'm sure it's been said on here somewhere, but honestly, I don't like the thought of Neville's parents being in the same institution where someone like Lucius gives handouts! He's shown, on a number of occasions, to throw money at people to get his way or have control over matters.
I don't get the impression that Lucius works at all...it seems more like he holds positions (like being one of the governors) where he can manipulate things to his will (like getting Buckbeak executed, getting Dumbledore suspended, etc.) of power and throws his money around...which he likely inherited. I suppose he could work somewhere he's actually expected to show up and perform tasks, but to quote Sirius, "I'll eat Buckbeak" if he does!