Despite a very silly name (and URL), Del.icio.us offers a great service - centralised bookmarking that you can get at from any machine, and by using keywords share others explorations of the web to find interesting sites.
By adding a simple link to your browser toolbar (IE, Safari and the various Mozilla flavours) you can instantly add any site / page you're visiting to you personal collection.
For OSX users who want to integrate this into their menubar, check MacUpdate.com for a number of integrated clients.
It's a great extension to the consensual sharing of information that the blogsphere provides, and ideal for people who hop from machine to machine to find things.
I'd love to see a bookmark importer that takes my Safari, IE and Firefox bookmarks, de-duplicates them and adds them to the repository, and maybe options to group them by high level folders (like normal bookmark managers in the various browsers) - maybe I should have a go at making the former myself, rather than just talking about it !