Here's how to sync your Things To-do database across multiple Macs using Dropbox for shared storage.
In some ways this is pretty obvious. I've just begun to use Things for To-do list management, but I wanted a way to keep the database sync'd across my desktop and my laptop. The Things website hints at this functionality.
All I did was point Things on both machines to point to my Dropbox folder.
[crarko adds: I haven't tested this one, as I don't use Things. A number of software developers are building in or using Dropbox for synchronizing data now, and it's especially handy with iOS devices.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20101201095023380