Passing along a good reminder by Tim Bulkeley who learned it from Suzanne McCarthy:
We've all probably faced at some time the problem of web pages that have simply disappeared or have been relocated. So what to do if you cited that page and now it's gone? Here is where WebCite is valuable. They state:
Click HERE to create a WebCite reference to your own material. Just making the web a better place...
WebCite®, a member of the International Internet Preservation Consortium, is an on-demand archiving system for webreferences (cited webpages and websites, or other kinds of Internet-accessible digital objects), which can be used by authors, editors, and publishers of scholarly papers and books, to ensure that cited webmaterial will remain available to readers in the future. If cited webreferences in journal articles, books etc. are not archived, future readers may encounter a "404 File Not Found" error when clicking on a cited URL. Try it! Archive a URL here. It's free and takes only 30 seconds.
A WebCite®-enhanced reference is a reference which contains - in addition to the original live URL (which can and probably will disappear in the future, or its content may change) - a link to an archived copy of the material, exactly as the citing author saw it when he accessed the cited material.