I think probably 30 or 40 sites mentioned when I bought Threadwatch. Some of them linked through to both sites, and many mentioned me in a good light, saying things like "of seo book fame" and whatnot.

Andy Hagans got a few links to his poker blog by mentioning that he bought it and for how much.

Recently Mick Sawyer posted a for sale ad for his black hat forums on Sitepoint. I don't think he was looking to actually sell the site, but just wanted to place a $10 ad while his site still had good mindshare from other recent mainstream media coverage.

Three downsides to placing legit public for sale ads are

• Some sites are harder to monetize under new ownership.

• If it is a community site some community members may be less interested in the community if they think of their content being sold to another owner.
• You let competitors know your market share data or profitability more than you may wish to.

Some sites can generate significant additional income with minimal effort, but if you buy a large community driven site you also have to factor the value of your time into the buy price.