An annual inspection had just been completed on the airplane and it was flown to the owner's location by a pilot-mechanic. The owner-pilot entered the airplane with the engines running and took control of the airplane from the pilot-mechanic. The owner-pilot then performed a takeoff on a wet, grass runway. The owner-pilot stated the airspeed stopped increasing at about 60 knots indicated airspeed, so he aborted the takeoff. During the abort, the airplane slid on the runway approximately 1,200 feet before the owner-pilot attempted a right turn onto a taxiway. During the turn, the left main landing gear collapsed and the airplane slid into trees located near the taxiway, damaging the left aileron. Examination of the airplane revealed dirt and insect larvae in the pitot tube. The pilot-mechanic stated he performed a preflight inspection, but he did not inspect the pitot tube.