I found this piece on The Several Habits of Wildly Successful del.icio.us Users to be a handy reference to some features that I have yet to discover/explore on del.icio.us. I just recently starting using the for: tag to push bookmarks that would be of interest out to other del.icio.us users that I know.
The inbox functionality sounds interesting:
The inbox is a mystery for many folks, but it’s a powerful tool. Your del.icio.us inbox allows you to track what’s been added to particular tags and to track what particular users have bookmarked. There are a couple of ways to begin using your inbox. You can click the ‘inbox’ link at the top of any page when you’re logged in and then click the ‘edit inbox’ link on the right side. Or you can just click the ‘settings’ link (top right corner) and then click the ‘inbox labels’ link under the ‘experimental’ section. Either way will get you to the same page where you’ll see a couple of text entry boxes. One is for adding USERNAMES and the other is for adding tags. If you know the del.icio.us USERNAME of someone you want to track, enter it in the ‘user’ field and hit the subscribe button. If you have a particular tag you want to keep tabs on, enter it in the ‘tag’ field and hit subscribe. Once you get the hang of this, you’ll fill up your inbox in no time.