Last week I installed XMarks Bookmark Synchronizer and I am impressed. I use 4 browsers (sometimes 6 if you include IE6 and IE7) and keeping my bookmarks in sync on all the browsers and across all machines is a real problem. I regularly use about 5 different machines so with my 4 regular browsers that makes 20 browsers that I would like to keep in-sync. XMarks makes this a breeze.
XMarks supports IE8 (probably IE6 and IE7 but haven't installed on these so don't know), Firefox, and Chrome. I don't believe that it supports Opera or at least I haven't found something that will work with Opera and XMarks yet.
When you install it do so on your browser that has your definitive or most comprehensive set of bookmarks and when walking through the wizard select the option to synchronize your bookmarks. On subsequent browsers you want to select the option to blow away your local bookmarks and use the ones on the server.
After that, any book mark you add, delete or modify on any browser will then be synchronized with all other browsers and across multiple machines.
Very impressed with this free utility.