An air dam is defined on the web as "A front spoiler mounted beneath the bumper and shaped to reduce the airflow under the car. Air dams can increase the airflow to radiators, reduce aerodynamic drag, and/or reduce lift," and from the same site, a spoiler is "An aerodynamic device that changes the direction of airflow in order to reduce lift or aerodynamic drag and/or improve engine cooling. A front spoiler is technically an "air dam" because it prevents air from getting under the car and increasing drag."
E28 Aerodynamics:
The US M5/535is air dam from twest:

It is a BBS/BMW unit, NLA, except by lucky finds. TT had a set a coupe years ago. It is this one:

Here is a detail pic of the front corner:

This is what the air dam looks like on a US bumpered car. This image from a BMW accessories catalog.

The rest of the 'kit' had these parts.

And that catalog had this rear spoiler in it.

Here is a close-up of the side skirts and rear valance, along with another BMW air dam for the car.

A BBS specific rear apron sold on the bay in Sept 2005 for $510, here are pics of that unit:

and

Zender had these parts available So if you want to know what the ZENDER looks like this is it!!!

This is the Zender side skirt on a dirty car.

and the Zender rear valance from the side

Alpina had an air dam that would do a fine job during a light snow fall. This indicating that it would push the snow, ah air out of the way instead of letting it go under the car. (Thanks Ted in T.O. for the link to this one)


I'll try to find more, including the Hartge air dam and post them as no one seems to be posting any pics of aero stuff for the cars. Well I did find the Hartge stuff and here it is:




As you can see, Hartge had two different rear spoilers, the polyurethane one and a wing that is painted like the car. The red car also shows the side skirts. Both air dams are without the cooling ducts that are in the center of the dam. These are similar to the ones in the 'is' air dam, except there are only two, one on each side of the center. These openings are in addition to the pairs of openings where the 'is' air dam would have the fog lights.
From the Aerodynamics post I have deleted, the Euro bbs posted by tannercore:






Posted by Aaron from Aus, BBS front spoiler like above:



and a Zender rear spoiler:
Jeremys front spoiler:

Jeremys pic of the BBS from the first set in the raw:

Jeremys pic of the Hartge Clone:

And two Alpinas from Jeremy:


The early shark style BMW spoiler from Jeremy photo collection:

The Racing Dynamics spoiler still available for 535 incl lights


From Zaprace in the UK, http://www.zaprace.com/orange.htm


Lightweight GRP kit for E28(modprod)as on the orange car:
Arches +75mm(3") set of four........440
Front Airdam depth 190mm/7.5".....248
Rear Aerofoil (E30 M3 shape).........265
above with adjustable blade..........335
above but "whaletail" one piece.....272
GRP lightweight Bonnet................246
GRP lightweight Bootlid................162
GRP Capacitor cover.......................82
GRP S38 Airbox............................195
GRP S38 Inlet manifold 6-branch....368
GRP Rear bumper.........................205
GRP Front lower vallance (chin)......228
Front Wings / pair........................328

Lightweight GRP kit for E28 as on the yellow car:
Arches +50mm(2") set of four.......405
Front Airdam depth 150mm(6")......228
Front Splitter................................92
Rear Aerofoil (E30 M3 shape)........265
GRP lightweight bonnet................246
GRP lightweight bootlid................162
GRP Airbox for M88......................185
GRP 6-branch M88 Inlet manifold...358
GRP Rear bumper........................205
all stolen from their site and plus shipping.
Post more aerodynamic parts and I'll incorporate them into this list.