Last year Alledia.com wrote an article about hidden advertising in Joomla extensions. Now they published an article detailing some extensions in which have been found hidden links: 

• Joomap and XMap: see our previous post.
• Joomlart's Transmenu: click to see how many sites are hiding links they don't know about.
• Smoothgallery : contains three hidden links, only one to the developer.
• Artio SEF : contained hidden links to European hotels and other unrelated sites.
• AddThis Social Bookmarking : hidden link recently removed after complaints.
• Fireboard searchbot : it places an ad for the developer as the first result in all searches made on your site. There is no disclosure but there a small option inside the mambot to turn if off.

Link | More information from Alledia