Once you get to the brokers list (link provided by Piphacker), most of the more frequently visited broker pages have a Broker Details section.
What you want to see is "ECN" What you don't want to see is an American Flag in the excluded countries list.
If you are in the US, going to the FPA's homepage will show you a list of a few brokers that do accept US clients. There are also some reviews displayed a little farther down the page. I think those are also for brokers accepting US clients. You'll have to open each page to see which of these claim to be ECN.