These are links which allow you to easily add your page to online bookmark services. Online bookmark services work like regular bookmarks, in that they remember pages you want to keep, but they allow you to access your pages from anywhere. Most sites also offer sharing facilites (see who has bookmarked what, find people bookmarking similar pages, etc.)
Yahoo! MyWeb is my pick of the services I've tried. It allows you to save a copy of the pages you bookmarked (useful when the pages go down), has fulltext search in your saved pages, and organises pages by tagging. MyWeb also works with Yahoo! Search, allowing you to save pages, and block others from future results.
del.icio.us is my next favourite. In the social/sharing aspects it is stronger, for example there are more users, simple URLs which make it easy to poke around, and watching other user's links seems simpler. There have been quite a few external tools built on del.icio.us, whereas MyWeb hasn't yet attracted developer interest in the same way. On the other hand, searching your own links seems stronger in MyWeb. Pages are organised by tagging.
Furl is a service I haven't tried yet, so I can't comment on it, but it looks worth a try. Backflip is the oldest of these services (I first tried it in 2000), and it shows (in a bad way). The interface is clunky, using folders rather than simpler tagging, and unless they have a major redesign it is not recommended. StumbeUpon takes a different approach, allowing you to rate pages up or down, and finds pages you might like, based on other user's recommendations. Again, I haven't tried it, but it looks interesting.
