If you do not know and use on-line social bookmarking site Del.icio.us, take a break now and go there. Currently owned by Yahoo, is Del.icio.us is a one of many social bookmarking tools out. Probably not the one with most features, not the one with coolest user interface - but pretty good one and certainly winner for me.
I have been struggling with the problem of how to keep my collections of URL's under control for several years. Yes - every browser has bookmarks or favorites, but if you use multiple computers daily basis (multiple means > 5), with actively accessing Web with more than single browser on each of them (usually with some subset of IE/Firefox/Mozilla/Safari/Opera), you can pretty much forget about browser's bookmarks/favorites - you will never have your bookmarks where you need them.
I tried several programs that "plug inside" the different browsers and provide bookmark repository for all of them. Some of them were pretty sophisticated and allowed even synchronization of the url-bases between different computer - but ... It still had two problems: remembering to do manual synchronization was lot of work, error prone and generally PITA and as these computers were running different operating systems (Windows, Linux, Mac) I never found good software that would run of all platforms.
The solution was obvious: keep bookmarks on line, in some sort of online repository. With this, you will have your bookmarks available everywhere, as long as you have internet connection. Which is the only situation where you actually need a bookmark. I know, it takes some adjustments to entrust your precious But let's be honest - there is not much use for your bookmark when you are offline - what would you do with them.
Thanks to Firefox pluging del.icio.us and bookmarklet for the same service in IE and Safari, I have finally all my bookmarks under one roof and accessible from any place. All 1225 of them - as of today. Why did I select del.icio.us and not e.g. Spurl, Furl, Magnolia, Clipmarks, Yahoo My Web ? I tried to use all the mentioned before, but del.icio.us had simples UI allowing fastest entry and it was backed by real company with revenue ( I was careful not to store my files on servers of a startup that can be out of business on a week notice). YMMV though - before making your own decision, see the Wikipedia page on social software, or the Social bookmarking faceoff article.
In the process of using it I have discovered the amazing aspect of the world "social" in social bookmarking. Unless you mark the URL as private, it is visible to everybody as well as it's classification (which is topic for another time). Clicking on the keyword/tag shows you all public links *anybody* classified same way. You can also select combination of keywords ... it is goldmine of information, co-created with over 500'000 of the del.icio.us users.
What is so great about del.icio.us is the easy way with which you can add new bookmark and tag it with keywords. The easy and user comfort is in my opinion most important feature of the online bookmark manager. I have stayed with del.icio.us exactly for this reason: it requires minimum amount of work to enter and classify the url. It makes bookmarking AND tagging URL so easy that you'll never more will have a reason not to bookmark a site ...
See for yourself, create an account on del.icio.us, get the Firefox extension and enjoy !
PS: You may also be intersted in this article on social bookmarking tools.