If you spend any time on the Web—and who doesn't?—it takes no time at all to build up quite a list of sites you want to visit again. Assuming you aren't just scribbling URLs down on a legal pad, you're going to want to bookmark those sites and the important pages on them. But if you bookmark them in Firefox, that doesn't do you any good on Chrome. Or if you bookmark them at home, that's no help at work. And online bookmark services like Delicious require that you log in to use them.
Xmarks looks like it may help you to overcome all these obstacles. You download it to your computer, but it then integrates with your browser—currently Firefox, Chrome, IE, and Safari (Mac)—to let you create bookmarks with any browser on any machine and share them with yourself on any other machine. You can create sync profiles, so that maybe your work bookmarks can be shared with your personal machine, but not vice versa. In addition, it also keeps a backup of all those bookmarks, just in case.
Xmarks is a free service. You just need to be running one of the supported browsers on Linux, Mac (OS X), or Windows (XP and later).