Xmarks is the name of an old plug-in for Firefox called Foxmarks. Foxmarks is used to synchronize bookmarks to/from different browsers. When the guys made plug-ins for different browsers I guess they changed the name. Now it supports IE, Firefox, Safari, and soon Chrome.
Anyway it’s super useful, it’s free and it’s relatively stable. When you somehow override the new bookmarks with an old version (I happens a lot) it’s easy to restore from the browser (Firefox backs up bookmarks regularly).
I’m using one account for me and it’s ultra logical to create one for the office – now everyone has all the new links.
Also if someone is too paranoid, the plug-in allows to use a custom (own) server, which, if I remember correctly, was just a Web-DAV (or FTP) directory with some privileges, but when I tried it (a couple of years ago) it didn’t work that well.