I have seen threads that had everybody post websites that sell parts related to our cars and thought it was a good idea. I'll share first and feel free to copy and add your's.
Mod Shops
http://www.khoalty.com/
http://www.ground-control-store.com/
http://www.bimmian.com/
http://www.Theretrofitsource.com/
http://www.ddmtuning.com/
http://www.jlevisw.com/
http://umnitza.com/
http://www.dinancars.com
http://stores.ebay.com/Eurosport-Gar...&_sid=44268313
http://www.bavariansoundwerks.com/ca...261/0/Catalog/
http://www.bavtoys.com/e46products.php
http://leatherz.com/
http://www.urotuning.com/
http://racdyn-usa.com
http://www.modbargains.com/
http://www.schmiedmann.com/ (Danish OEM and Performance)
http://www.cecwheels.com/ (AC Schnitzer)
http://www.evosport.com/product/bmw/
http://www.wheelsto.com/ (Hartge/3Ddesign/GruppeM/Akrapovic exhaust/Macht Schnell)
http://www.oldstock4less.com/
http://www.purems.com/products/product.php/II=77 (BBS Center Caps)
http://www.ind-distribution.com/ (Exhaust Systems)
http://www.redlinegoods.com/SteeringWheelWraps.php
http://www.masonengineering.net/
OEM Parts & Drawings
http://www.getbmwparts.com/
http://www.realbmwparts.com/
http://understeer.com/
http://www.realoem.com/bmw
http://www.eurosportacc.com/
http://www.penskeparts.com/fiche.aspx
http://bmwfans.info
http://www.bmaparts.com/index.php
http://www.bmwmercedesparts.com/
http://www.gutenparts.com/
http://thebmwpartstore.com/
OEM and Aftermarket Parts
http://Www.ecstuning.com/
http://www.carparts.com/
http://www.Pelicanparts.com/
http://www.Autohausaz.com/
http://www.bimmerworld.com
http://www.rmeuropean.com
http://www.bavauto.com
http://www.bmwpartssource.com
http://www.europeanautosource.com/index.php?cPath=67
http://www.bmpdesign.com
http://www.coverking.com/
http://www.rockauto.com
http://www.akgmotorsport.com/catalog/catalog.php
www.furiousmethod.com
www.fcpeuro.com
Mod Shop & OEM/Aftermarket Parts
http://www.turnermotorsport.com
http://www.euro-dyne.com/bmw/3-series/e46/
http://www.vacmotorsports.com/
John started the thread. The above is the accumulation of what we have up to this point.