del.icio.us just added a cool feature: you can now take the intersection of the tags you assign, which may simplify my tagging in the future. For example, I have a 'blogs' tag and a 'mozilla' tag, so if you select both you get my link to asa's blog (mozilla developer). The UI works fairly well too. You click on the first category, and then shows you a list of related tags that you can do a intersection with.