Autotrader - reasonably good, interface is fine, loading time can be on the long side (even with a 1.5 mips connection -- that's what I have at home).
Autoweb - straight forward interface.  Fairly good stock of cars.  I didn't like the fact that one needs to enter personal information in order to learn more about a vehicle;  I suspect that this is related to tracking transactions.
Calling All Cars - Homepage load is very slow.  One is forced to sift through all autos of a particular make; the website needs more filtering options when searching for cars.
Classifieds 2000 - Comprehensive website.  One can buy a broad range of products and services at this website (e.g., office, tickets, general items).  The automotive section was easy to use and organized reasonably well.  One can also connect to Autobytel through this website.
AAAA Auto Exchange - Nothing too fancy.  Not too bad.
AutoParkUSA - Do not park, do not idle, keep moving.
Autovantage - Lots of product and lots of miles.  Prices rarely listed for autos with lower mile or those listed by a dealer.
Autobytel - They do a reasonably good job at getting good product online.  You won't find a "steal;" I sense, however, that you will get a decent car.
Honda - Honda website claims that they will direct you to certified cars...this was not apparent to me.
AutoNation - available in select areas only.
Ford - Ford preowned website.  This is currently being tested out in select areas.  Ford is fairly aggressive in using the Internet to reduce friction and create a direct relationship between itself and its customers and consumers.
Jaguar Registry - Even if you are interested in a Jaguar, go elsewhere.
DealerNet - It would help if the prices were listed (in fairness, I did find a handful of cars where price was listed).
Stoneage - These folks from the Motor City will give you enough product to check out anywhere.
VehicleNet - Their ad states that  "there's never been anything like it." I hope that I never visit this website again.
e-auto - Messy website...still, lots of good stuff here.
PriceAutoOutlet - Off lease cars at reasonable prices.  Can be hit or miss depending on where you live.
Cars.com - Essentially, the newspaper classifieds online plus other helpful auto information (but, I know Edmunds.com, and this is no Edmunds.com).

This list is by no means comprehensive.  Most search engines will steer you to particular auto websites (I use the AltaVista search engine primarily).  Nevertheless, I believe that this list would be serviceable to most people.

Bart Weitz visited Boston University today and presented some of his research on intelligent agents in marketing.  The presentation was first rate and mind expanding.  I had a wonderful time visiting with him and listening to his presentation.