I love del.icio.us. I have to admit at first I did not see the benefits of it (like others that I've talked to), and each time I saw it I just closed it. It wasn't until I actually started using del.icio.us that I started to appreciate it. In my opinion the most useful features are the RSS feeds of the most popular URLs, and the inboxes of other users.
Our corporate intranet is huge. There is just so much information on it, you can't easily find everything you need. When people in our department do find information they need, it rarely gets shared, and a lot of time is spent by other team members looking for the same information. This led me to thinking "we need a del.icio.us type service on the intranet".
As a lot of our information is private, we can't use del.icio.us. I began thinking of coding a clone for use internally. A couple days after deciding to code my own rjbs released Rubric 0.00_01. I installed it and began working with rjbs (where working means complaining that things aren't working right) to fix multi-user use. The addition of open registration has allowed me to deploy Rubric to our department without burdening me with user creation.
While Rubric started out as a clone of del.icio.us, it's grown into something more. One of the new features is the ability to create a note, either attached to a URL or on it's own. Add to this the ability to require a user id and password before any information is displayed, and it makes for an excellent note management system.
I used to have a web application on my home intranet to maintain my wife and my notes (recipes, technical tips, code snippets, etc) written in PHP (that broke after an upgrade to PHP). Rubric has now taken it's place.
Lately, I've noticed that del.icio.us and nutr.itio.us (a popular interface for posting to del.icio.us) have been experiencing problems. I haven't been able to use nutr.itio.us since before Christmas, and I regularly experience problems with del.icio.us (connection refused, 500 errors, etc.). I assume these are to do with the rapid growth and popularity of both services. Luckily I now have my own Rubric web bookmarking application.