Ray Ozzie mentioned an interesting variation of the RSS protocol. RSS is essentially a one-way synchronization protocol. What it would take to change the RSS standard in a very minimal way, such that it will work for two-way replication. A simple idea, with very deep implications. The new RSS is now called SSE: Simple Sharing Extensions. You need relatively little programming work to implement it.
In the preliminary discussions, Dave Winer immediately pointed out another interesting extension, for something he wanted for some time: how to cross-update OPML files.
Here is the SSE draft and a FAQ.