I read this Jamie Zawinski essay last year, but it’s worth another look.
The trick you want to accomplish is that when one person is using your software, it suddenly provides value to that person and their entire circle of friends, without the friends having had to do anything at all. Then, later, you pull the friends into the fold: if one of them starts using the software, they become their own hub, and get the benefit they have already witnessed from a distance.
The reason I landed there was because yoga classes are starting all over the Upper Valley, and I thought, It would be monster cool if there were a website where yoga studios could enter their class schedules, and publish them to a centralized (and, natch, localized) calendar where I could view them all together. And it would be even cooler, if that web app could generate iCal-format downloads that I could import into my desktop calendar. Ten seconds later I realized that yoga classes were a specific example of a much more general use-case with very realistic and widespread needs (school sports come immediately to mind, in addition to live music).
What I want: A consumer-grade website where I can “login,” and “create,” “edit,” “delete,” “search” or “browse” for one or more “topics” “within X miles” of “zip code,” view that in a “list, week, or month-view calendar,” “select items of interest,” and “generate iCal” (and other standard) format downloads of that selection.
If you know of such a service please tell me so I stop designing it when I have client work to do. Related and useful: hCalendar and other assorted microformats.
Update: Doug asked about standards. Here are the links that probably matter most. I’m sure there are other standards, I’m just taking an open-format Mac-centrc approach.
* CalDAV on Wikipedia.
* CalDAV resources from the OSA Foundation.
* Calconnect consortium.