Plain water because it is free and less polluting. I don't do laundry very often anyway. I guess my only secret is i leave my "dirty" clothes soaking in a bucket in the closet, and i don't wear white clothes. A few years ago there were supposedly these "magic" laundry balls which could replace detergent and they seemed to work, so even a bunch of conventional groups like Consumer Reports tested them against detergent and as a scientific control, plain water. And they were suprised that the plain water control was just about as good. Some times detergents contain brighteners which are like ultraviolet poster dyes so they glow and create artificial cleanliness, but they are probably not very earth friendly. Here's a more humorous discussion: http://www.straightdope.com/classics/a4_007b.html
Just a scoop of caustic soda and a handful of washing soda crystals in a front loader does a surprisingly good job.
