AutoCAD Add-Ons - Plant and Piping:
Trace Plant Designer
http://www.traceindustry.com/en/tpd/index.asp
PROCAD
http://procad.com/
PipeDesigner 3D
http://www.quickpen.com/solutions/cad/pd3d/
FLI (Flow Logic International)
http://usa.autodesk.com/adsk/servlet...linkID=7770993
FlexCAD
http://www.flexcad.com/
CADWorx
http://www.codecad.com/CADWorx.htm
AutoPLANT
http://www.bentley.com/en-US/Promo/AutoPLANT/
Cadpipe
http://www.cadpipe.com/
Mech-Q
http://www.asvic.com.au/