A personal favourite of mine is the National Library of Australia's newspaper beta. This site "allows access to historic Australian newspapers digitised as part of the Australian Newspapers Digitisation Program."
I like the:
*user comments, tags and text corrections
*open source codebase
See it in action here:
Voting on Ideas
Vote for your favorite ideas by clicking on the up arrow.To undo an upvote, simply click the arrow again. This second click removes your vote.