I use the Mozilla web browser on a few different machines, and its lack of roaming profiles/bookmarks is a source of annoyance. When I bookmark a site on my laptop, I want that bookmark to be available on my desktop machine. In order to achieve that I use a kludgy network of scripts, CVS, crontab entries, and the bookmarksync application.
Bookmarksync (http://sourceforge.net/projects/booksync/) is a tiny little program that takes as input two different bookmark files and outputs the combination of the two. It works just fine in all respects, except that it converts tab-group bookmarks into bookmark folders.