THE AIRLINE TRANSPORT PILOT DEPARTED AT NIGHT IN A FOUR SEAT AIRPLANE WITH A TOTAL OF FIVE PEOPLE. HIS PREFLIGHT WEATHER BRIEFING NOTED IMC ALONG HIS ROUTE OF FLIGHT ASSOCIATED WITH A COLD FRONT. FLIGHT PRECAUTIONS WERE ISSUED BY THE BRIEFER FOR MODERATE RIME AND MIXED ICING ALONG THE ENTIRE ROUTE OF FLIGHT. HE ELECTED TO DEPART VFR AND AIR-FILED AN INSTRUMENT FLIGHT PLAN. HE RECEIVED SEVERAL INFLIGHT WEATHER BRIEFINGS GIVING LOW IFR CONDITIONS AT HIS DESTINATION AND SURFACE TEMPERATURES JUST ABOVE FREEZING. HE ELECTED TO CONTINUE THE FLIGHT AND THE AIRPLANE BEGAN TO COLLECT ICE. HE WAS UNABLE TO MAINTAIN ALTITUDE DUE TO THE ICE ACCRETION AND ULTIMATELY DESCENDED INTO TREES AND TERRAIN. THE AIRPLANE WAS NOT CERTIFICATED FOR FLIGHT INTO ICING CONDITIONS, AND WAS NOT FITTED WITH ANY AIRFOIL/PROPELLER DEICE OR ANTI-ICING EQUIPMENT.