I signed up for del.icio.us almost a year ago and sure haven't been using it. But thanks to Kirsty Hall's post and link to this wonderful video on Common Craft, I am much more informed. The video explains how and why to use sites like these in simple, easy-to-understand English. And it's short and entertaining.
Even if you don't use these sites for "social" bookmarking, I can see it's a great way to arrange your favorite sites.