That said, there are still plenty of great options in the ETF market:
Corn: $CORN
Soybeans: $SOYB
Wheat: $WEAT
Sugar: $CANE
Natural Gas: $UNG & $UNL
Gasoline: $UGA
Oil: $USO & $USL
Copper: $CPER (or go with Copper Miners $COPX)
Gold & Silver: $GLD, $SLV
Platinum & Palladium: $PPLT, $PALL
Agribusiness ETF: $MOO