I use Planet detergent and have had really good luck with it. It's available at Kroger, but I buy it on amazon.com and get free shipping by buying over @25 worth at a time. Here's a great link for cloth diaper detergents too:
Another vote for Planet.
We use Planet too. I like that I can find it just about anywhere. I think it's usually about $8-$9 and we just had to buy our third bottle in almost 14 months. Not bad, I think.
www.PinStripesAndPolkaDots.com has good info on different detergents too.