I found this one while doing my daily browse through Digg. It's a tool from Google that lets you sync multiple Firefox installations in realtime. I personally have 3 different machines... nowait, 4... that I run Firefox on, and being able to seemlessly keep my bookmarks, cookies, form elements and tab/button bar layouts synchronized is a total win, not to mention having an off-site backup of all these goodies.
I recommend starting this on the machine you have your most complex and involved bookmark mechanism on, as when you add a new machine to the mix, it appears to import your saved bookmark collection from Google Sync, and then synchronize. So the first one in should be your largest. I probably have 400 bookmarks in my setup, organized into dozens of categories.
As found via DesktopLinux.com.
I thought that looked pretty neat too. I used to use the bookmark synching plugin for FF but it doesn't work anymore.