Yahoo! Bookmarks

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Yahoo's MyWeb is now (accompanied/replaced?) by Yahoo Bookmarks, which has cached copies of bookmarked pages, thumbnails, tagging and recommendations.

Cached copies of pages have unique URLs, but I suspect it's still only storing one copy of a page per user, so you still need to append a timestamp to each URL to use this service for archiving and citing web pages. Also the URLs are quite opaque: I don't think there's a way to construct them with a page URL and a date, like you can with WebCite or archive.org.