Out of control outgoing links
Okay, I already knew that I hate and detest the new-ish Facebook practice in which you don't just navigate to outgoing links; you get taken to a Facebook page with the site you were trying to visit loaded into a frame, and with a Facebook toolbar at the top of the page.
And now, not only does Digg do it too, but the sites are so dumb that they're double stacking these stupid toolbars.
This is crap primarily because it breaks basic web behavior: the URL on your web browser no longer represents the page you're viewing, or trying to view. Big juju nono. We've known this since 1996, people!
Also, I don't want to send someone to http://www.facebook.com/ext/share.php?sid=89405539347&h=1e79i&u=2fnBr&ref=nf. I want to send them the link that's www.youtube.com so they know where they're going and what they're going to get.
The one case where Google does something like this -- a frame with another site inside its toolbar -- is when a user clicks on an image search result. That frame bothers a lot of site owners that Google links to, but it was a necessary tradeoff because image search is kind of sketchy and occasionally unreliable. Users click on an image result and get taken to pages that no longer have the image they were looking for, and the toolbar exists to help them identify the image they wanted -- NOT as a marketing-sticky thing for Google. And even though it's often beneficial, it still confuses a lot of users.
Note that mismatched content and URLs is a usability problem for Google Maps. The URL that many users copy from their browser address bar usually doesn't correctly represent the state that the user's in due to the way the site's AJAX works. AJAX or FRAME, it's the same problem.