post-2008 Energy Star certified appliances are what you are looking for. Appliances from 2008-2011 were 'self-certified' and there was some abuse, so either check the specific model to ensure or buy something from model year 2011 or newer.
Beyond that, how you use it matters far more than the appliance itself (e.g. washing on cold and hang-drying most items will have a far greater impact than buying the most efficient washer/dryer combo and washing on hot and tumble-dry). likewise changing your freezer thermostat to ~10ºF vs -4ºF.
As always, buy used on Craigslist. Given their poor portability and frequent kitchen remodeling, there are tons of deals on recent-model appliances, often for < 1/3 of buying new. Purchasing new appliances has the biggest carbon footprint - we need to keep slightly used appliances from going into the landfill prematurely (which, sadly, is the fate for many good appliances).
