For me it would have to be Disqus. I got lazy, needed replies/comments quick... and it violated the policy I have that content should NOT be delivered via JS only. That and it is massive to the point of by itself being 30 to 50 times the size of the pages it was going into.
BUT I allow it as it's only for comments/replies, not the actual site itself's content. Scripting off the site's content is still there, you just lose the comments.
Still feels dirty.