I am a big fan of del.icio.us. Since uploading my bookmarks (then a reasonable 645) to del.icio.us last year, I have never felt a need to return to storing favorites in a browser. Being able to access all my bookmarks from any device with web access is simply invaluable.
The Firefox add-on Tags Everywhere uses del.icio.us and is very convenient when adding many new sites at once. I prefer using the Firefox browser outside of work. (There is no access to Firefox at work.)
Like other web 2.0 tools, del.icio.us bookmarks can be accessed from other places besides the del.icio.us website. I am using the del.icio.us application in Facebook and friends can access my bookmarks from my profile.
I enjoy sharing bookmarks with friends and colleagues, too. At the same time, I like the option to keep certain bookmarks private.
Recently I have set up several RSS feeds for different tags in del.icio.us. I have found this to be a better way to keep tabs on a topic than by simply using the subscriptions feature.