Demo iFrames
With the Ui.Vision for Chrome webextension the Selenium IDE selectFrame command is used to switch between the main page and the page inside the frames/iframe for all HTML/DOM based commands (e. g. CLICK and TYPE). For visual automation with XClick and XTYPE, iframes are nothing special and no additional command is needed, except some little trick for scrolling.
Embedded Google Forms iFrame
iframes are often used for embedding content from other websites.
Youtube iFrame
Embedded Youtube videos use iframes.
Twitter iFrame
a9t9 tweets inside an iframe.