In light of all the "X is a fraud" threads I've seen recently, I wanted to share as many of my experiences as I can remember off the top of my head.

I've had great experiences with these private sellers on Bimmerforums:

BAADDOG
Shibumi
///Mista ED (was a trade)
Kosta


I've had great experiences purchasing from these stores (Bimmerforums contact, if known, in parentheses):

Understeer (bimmer95)
Bimmerparts (CMUZYY)
Bimmerworld (jamesclay)
Bavarian Autosport
Turner Motorsport
HMS Motorsport
BMP Design


I've had great experiences with pre- and post-purchase customer service from these stores/companies:

Understeer
Bimmerparts
Turner Motorsport (exceptional!)
Wheelskins (exceptional!)


I've had what I consider less than satisfactory customer service from these stores:

Active Autowerke
Crutchfield

Here are some of the "good guys" I've purchased parts and supplies from:

Forum members:

Preppy
Drea///M3

Vendors:

Turner Motorsport
Bavarian Autosport
The Ultimate Garage (BF contact: SteveD)
Zionsville Autosport
The Tire Rack (BF contact: Jim@tirerack)
Vines Automotive
Griot's Garage
Car Care Specialties

I recently placed an order with Mpact Motorsports and hope to add them to the "good guys" list soon.