I particularly like Stack Overflow as an example here. You can visit Stack Overflow and ask a question, all without having to create a profile. You are given a name, given a profile, and given most of what a regular user gets.
As soon as you do decide to sign up "for real", your temporary profile is turned into a real profile. The biggest difference being that you're not cookie-based (you can log in to the profile from other machines).
Despite Jeff Atwood complaining loudly about how so many users have stayed cookie-based for so long, Stack Overflow has gone to great lengths to make sure their site is completely usable with a minimum of hassle, which makes it a much better site in my eyes.