What is Social Bookmarking? If you know the answer… go to the next paragraph. If you don't, it's simply a way for users to save and share bookmarked links with one another. Del.icio.us is a nice service that allows you to share and ‘tag' the links. Tagging your links allows you to easily find the links you're looking for using the Del.icio.us interface.
I'm not a huge fan of the Del.icio.us website, but I am a fan of all their extras. You'll see a WordPress widget for Del.icio.us loaded on my main page (comes from Automattic with the sidebar widgets plugin). As well you'll see it integrated in my feed using Feeburner's Link Splicer.
My favorite use of Del.icio.us, though, is the Firefox Plugin. Notice in the image below, I've added a “Tag” button in my address bar. When you click that button, it pops up a nice form you can fill out for tagging and saving the URL to your Del.icio.us library.
One cool little feature you may not be aware of: If you highlight some text on page and then click “Tag”, it will automatically paste the highlighted text in the Notes field! Nice little feature and timesaver! Here's a screenshot below: