Google instant previews
google.com
google.com
Theirs customizes the preview type depending on the content. For example, links to stock information shows a little quote and graph preview.
I'd say that the snap.com UI probably falls into the "Hover and cover" UI Antipattern (as defined by Bill Scott)
http://www.slideshare.net/billwscott/design-anti-patterns-ho...
(slide 36 onwards)
I highlight while I'm reading, and it plays against that by distracting me with the popup. The article says that it comes up from pressing the icon, but it also includes the text. I'm leaving it enabled though to see if I benefit from it like I do Instant.
That said, I'm still unsure about Google Instant because I'll see my result then keep typing and fly by it having to backtrack and see where Google gave it to me.
Other notable notes: they don't render flash in the previews and they let the hovers cover up the right-hand ads.