Archive.is (previously named archive.today) allows you to "take a photo" of a webpage that will always be available even if the original page changes or is deleted.
Archive.is saves both a textual and graphical (1024x768) copy of the page (for accuracy) and provides a short URL. Unlike other services, it can also archive the following type of pages:
* hashtag pages from Web2.0 sites, e.g. https://twitter.com/# !/search/%23nowplaying
* 404 pages
* pages forbidden by robots.txt rules
* httpS pages More Info »
Archive.is has opaque URLs for individual snapshots (e.g. http://archive.is/n9Yb ) but transparent URLs for all public snapshots of a given URL (e.g. http://archive.is/http://twitter.com/%23 !/medvedevrussia)
Scripts are disabled, reportedly to prevent pop-ups or malware, but in practice this results in legitimate sites not functioning after archiving (e.g. Google Maps), which can often be an intended benefit (where would the map image come from if you panned left in Google Maps?)
Archive.is also lists a live feed of all URLs archived, so your archive may be public; however, since archive.is doesn't require a login, and the URL you archive must be public to begin with, this is not a privacy concern.