Squidoo

Squidoo is another article writing site. I haven't personally done much with this site.  I do like the look and feel of the squidoo site but for whatever reason I haven't been very motivated to work with this site. It was stated by Seth Godin, an author. It calls it's pages lenses and there are well over 1,400,000 lenses. You write the lense much like an article. The passive income part is that you can add Adsense, Amazon, Ebay, CafePress and affiliate offers to your pages.

Earning money from revenue that the company receives from everything except Adsense is split like this: they give 5% straight away to charity, they keep 45% to pay salaries, bills, and keep the site running. That leaves 50% which they give directly to the lensmasters (that's you) for your good work. You can then either take your royalty payouts in cash via PayPal, or donate your royalties to one of a few hundred great charities. 

Ad revenue (Google and other ads). This money goes into a pool, so as to avoid clickfraud. Half of the pool distributes out to lensmasters, on a PayRank scale that is based on each lens's average LensRank and traffic, and the number and quality of lenses in the lensmaster's account.

As you build up traffic and lenses your earnings increase. Writing good quality lenses could provide a nice passive revenue source that will be around for a long time. Squidoo has been around since 2005, and got rolling in 2006 so it is not a fly by night adventure.