I have just found another couple of sites that look useful and innovative. Snipsly and Jevitt combine features of social bookmarking sites and article directories.
You can post do-follow links to blogs, articles, and other content. You can make the surrounding text a complete article in itself, or as short as a few sentences.
I quite like this idea because sometimes you want to post a little nugget of information, or a pithy bit of commentary, rather than a comprehensive 500 word article. These sites allow you to do so and still get some SEO benefit.
They also have Adsense revenue sharing enabled, so you could make a bit of money as you do so as well.