This is a short entry about sites that use "Lazy Registration," or registration/sign up processes that often don't require any information from the user while they try out a site, until they decide to commit to using it. In some cases, sites bypass signup altogether, and in others, users are permitted to register once they've decided to go beyond simply "testing out" a site or application. As noted, the process of "getting your feet wet by trying," most likely leads to more sign ups because of the frictionless experience.
The article includes a collection of examples including StackOverflow and Drop.io. I personally like Soup.io's process as well and might include it among the good examples here. Check out all of the examples.