Taking lazy registration one step further and going with no registration at all is a cool concept, as well. If you can center all of your functionality around email (or something else that already provides auth), it's possible.

When App Engine was released, I made an app called Lunchstr (http://lunchstr.appspot.com) just to play with GAE. The app isn't very useful, but the no-login, no-registration philosophy was kinda cool.