I’ve just discovered a new site called Geni.
In all web application requiring users to sign up before they can use it. One big barrier is to get the user to actually sign up in the first place. By instantly giving her some kind of gratification, like in the Geni example, you significantly lower that barrier. The user only have to enter a minimum, and in this case very relevant, amount of information before they can reap their rewards. No bloated forms to fill out. You just get started right away.
All other information you need from the user can be added later, when the user’s already up and running. After having invested time and effort in the application she is much more likely to give away more information about herself.
Do you have other examples of web applications who has managed to make the sign up process almost invisible?