Simple Sharing Extensions
It's about time that RSS was leveraged as a means to synchronize data sets. As Ray Ozzie describes in his latest blog, calendars really need to co-operate.
Simple Sharing Extensions for RSS and OPML is a great start. It allows multiple applications to reference each others RSS feeds and keep themselves up-to-date when items are changed. FAQ here.
Worthy of note: Microsoft have released this (after validation with Dave Winer) under the Creative Commons license Attribution-ShareAlike so start adding this to your apps!