For as long as I can remember I've been washing my clothes with cold water. People seem to think that you "have to" wash your clothes with warm/hot water in order to get them clean. I'm here to tell you that is not the case. I've never had a problem getting the clothes clean with cold water. If you disagree with me then try it out yourself. Go ahead, I dare ya.
The reason to use cold water is that it saves energy. About 90% of the energy used when washing clothes in a top load washer is to heat the hot water. So if you wash your clothes in cold water you can cut your energy use by up to 90%.
The Seventh Generation company has a calculator thingy that will let you estimate how much you can save if you wash with cold versus hot/warm. For me and my wife we only do 3 loads a week in our washer and we have electric hot water. Their estimator says we save about $34.89 a year in electricity by washing our clothes in cold water. A large family that does 10 loads a week could save $116 a year. Of course the exact savings depends on the washer in question, the cost of your electricity and how often you wash clothes.