Scott Rafer gave me the tip on social calendar, 30 boxes. There are atleast 15 startups working on socializing your calendar. These guys seem to have the fastest growth, though. From their blog:

We have taken to allowing signups during the evening as we roll out some more servers. This experience has been exciting and surprising as well. We had a hunch that people would like some of the things we were up to, but didn't think that it would happen so fast. We are trying to reply and react to forum posts as well. Thank you to everyone contributing and we can use community assistance in replying to questions from newbies.

They have tags, syndication, remote skinning, an api and sms. These were the guys that were behind webshots, which now drives most of the traffic behind the CNET network. So, I'd imagine they know what they are doing.