I've been looking to buy a new car, and these are the ones I've looked at found useful, despite not doing side by sides:
www.consumerreports.com
www.edmunds.com
http://www.autos.msn.com/
http://www.autos.msn.com/
http://www.autobytel.com/
http://consumerguideauto.howstuffworks.com
And, since I still need help on negotiating, I thought the below was particularly helpful:

http://www.wikihow.com/Buy-a-New-Car

Good Luck!

Edmonds

Side impact airbags are going to weigh heavy on my next car purchase.

http://www.consumerreports.org/cro/cars/safety-recalls/carcrashtest/crashtestvideo.htm

The list offered by rowenaz is great. Only addition I can make is aaa.com

beep beep .com

There are a lot more:
http://autos.yahoo.com/car-compare/
http://www.autotrader.com/research/compare/index.jsp
http://www.fueleconomy.gov/feg/Find.do?action=sbsSelect
http://www.fueleconomy.gov/feg/Find.do?action=sbsSelect (nice site to compare on gas)
http://cars.findthebest.com/
http://www.autocomparison.com/compare-cars/
http://www.motortrend.com/new_cars/comparisons/

Just google ‘compare cars’

just saw how old this question is.. Sorry I’m new here;)