I find iFrames messy, and only will use them as a last resort. To answer your question, I have used them in instances where security or lack of an API makes it

I find iFrames messy, and only will use them as a last resort. To answer your question, I have used them in instances where security or lack of an API makes it impossible to do otherwise (an example was for a chamber of commerce site using web widgets from a company that provided them as an add-on to a PC based SaaS product). I have also used them when there was not a budget to recode a a stand alone web gaming application that needed to be placed into Facebook. In a perfect world, I would never use them when starting from scratch and would search for an API or other alternative.