I'm particularly fond of WeeWorld - I love the illustration style, the feel of the interface and the thinking behind it - that we can all have an online identity that follows us from place to place. The concept of an avatar has been around since the nascent beginnings of web forums, but to have a community of identities that allow for customization, represent the interests and self-structured concepts of each other, and create a common visual language is really quite special.

WeeWorld already has support for Skype and a few other communities, plus you can set it up to be part of your email signature in Outlook or major online mail programs (with the exception of Gmail). I believe they're coming out with more cool uses for the system in the near future, too - definitely a "Web2.0" trend to watch.