Apple's iChat had a wonderful feature called iChat Theater--also present in the Mountain Lion version of Messages, just without the "iChat" in its name--that let you share photos, PDFs, or Keynote presentations during a video chat. I used it countless times to give virtual presentations: An audience watching on a projection screen in a remote location could see video of me alongside my Keynote presentation, and I could see video of the audience plus a miniature view of my presentation (and anything else on my screen, such as my email or a Web browser).
