A CARGO TRANSPORT AIRCRAFT WAS TAXIING TO THE CARGO RAMP TO UNLOAD ITS CARGO WHEN THE LEFT WING TIP STRUCK A LARGE PORTABLE MAINTENANCE STAND. THE MAINTENANCE STAND HAD BEEN LEFT UNATTENDED NINE FEET BEYOND THE MARKED TAXIWAY CLEAR ZONE. THE OUTBOARD 18 INCHES OF THE LEFT WING TIP WAS TORN OFF BY THE COLLISION, WHICH ALSO CAUSED A 75 TO 80 GALLON FUEL SPILL.