Fast will Also tell you what options your vehicle came specced with.
EPC-Data is good but when searching parts it can be a bit confusing cus it's pretty vague with its descriptions.
For instance that link Simon posted up
20010 - TUBE ASSY-EXHAUST,FRONT20018 - TUBE ASSY-EXHAUST CONNECT,FRONT
20020B - NUT
20020BA - NUT
20020BB - NUT
20100 - MUFFLER ASSY-EXHAUST MAIN
20651M - MOUNTING-EXHAUST,RUBBER
20651MA - MOUNTING-EXHAUST,RUBBER
20651MB - MOUNTING-EXHAUST,RUBBER
20651MC - MOUNTING-EXHAUST,RUBBER
20651MD - MOUNTING-EXHAUST,RUBBER
20691 - GASKET-EXHAUST
20692M - GASKET-EXHAUST
20692MA - GASKET-EXHAUST
Whereas in Fast you goto General Index, Engine & Fuel & click on the picture of the exhaust and it'll give you...
Where you can click on the part codes for which gasket/hanger/nut you actually need and get the part number from it.