The Wayback Machine hosted by the Internet Archive preserves snapshots of websites. It is frequently the only way of retrieving information on web sites no longer on the 'net. It is an indispensable tool for digging up information on abandoned Tcl projects and recovering lost screenshots and downloads. If a link to something here on the wiki is broken because the site has disappeared, then the info may still be up in the archive.
lexfiend 2006-02-16: It's also useful for finding older versions of existing content (i.e. time-travelling). To find all archived versions of, say, http://www.scriptics.com/ , simply go to http://web.archive.org/web/*/http://www.scriptics.com/ and take your pick.
Lars H 2010-06-10: I just noticed that (the current) WubWikit seems to be confused by the second http: in links such as the above if put in brackets:
A work-around is to %-encode the colon, writing http%3A, like so: