The coolest thing about open communities is the unexpected idea that comes of all the interaction. First, take a status message tool with a bunch of active users. Second, have one of those new users be a programmer. Third, make sure the host site releases an API to allow other developers to plug into the community. Fourth, be inspired by some of the things already out there. Bam! TwitterMosaic.
Dandelife, the Facebook application
While I am certain there are longtime users of Facebook who look at the recent opening gates and extensibility as the end of the Golden Age, I am not one of them. By this time tomorrow, DandeLife will take a shot at leveraging those twenty million members with a Facebook application. Perhaps by Friday, they will have 6 million members.