Yahoo has a nice calendaring option that integrates into MyYahoo, and replicates with PDAs and Outlook thanks to Intellisynch. You can publish your calendar to a specific url. You either have to open your calendar to everone (and Yahoo calendar urls are easy to guess, which makes sense usability-wise), or you can restrict it to designated "friends." Now the problem is, "friends" are people that have a Yahoo id. So you either have to expose your calendar to everyone and their dog, or you need to convert people to opening a Yahoo account, which is probably their goal, but is self-defeating because you're probably not going to even bother trying. We need interop between logon/invite systems, for fear of limiting real usage of web applications.

It's quite cumbersome to bother with a different ID for each community you want to join. And if every player wants to be the gatekeeper, we'll be back at the beginning (having a Passport + Liberty Alliance + Yahoo + whatever else ID).