Website / Wayback Machine
The Wayback Machine is a section of the Internet Archive (and should not be confused with the Internet Archive itself). It is one of the most well-known and popular features of the IA, as it allows people to see past versions of web pages — in other words, it's the browser version of a Time Machine.

Here's how it works: after accessing the WM website through the link at the top of this page, users can paste a URL address on the input-box next to the "Take Me Back" button, and then, after clicking said button, the user is shown a calendar-like list of archived pages (provided there are any). Dates written on blue dots are links to versions of that particular page archived on that particular date.

It's not 100% reliable — sometimes the particular page or image you remember most fondly will turn out to be missing from the Wayback archive. The site also follows the Robots exclusion standard, so if your favorite website blocks the Wayback Machine from archiving it in its robots.txt file, then it and its content becomes inaccessible to the public (infuriatingly, if the domain is taken over by a cybersquatter who then implements a robots file, it will also block you from seeing the earlier, legitimate versions of the website). The IA also takes no chances with the law, and so all requests by the copyright owners to remove data from the Wayback Machine are immediately obeyed, so it's not necessarily the best archiving service to stymie an Orwellian Editor. A Canadian mirror exists, and has suddenly become more relevant due to fears of Internet censorship in the United States.

It's possible to use the IA to instantly archive any given webpage, too — simply go to the address[url_of_the_webpage] to save the newest version in the archive.

By the way, our very own TV Tropes wiki is represented, and preserved, in the Wayback Machine as well — and quite nicely, in fact. So if you want to see the wiki in its earliest days, give it a spin. It also preserves articles that have been subjected to Example Sectionectomy or removed outright. This link leads to the results for the TV Tropes homepage in the Wayback Machine; changing the URL posted to that of the article that interests you, you can access most of the site as it used to be. It's also a good way to show (and not tell) exactly why certain articles are now perma-redlinked.

Also, on a quick side note, while the Wayback Machine is the most popular tool of the Internet Archive, it's always good to remember it isn't by far their only good feature. For example, the IA also has a video and public file hosting system which several major archiving initiatives such as the Archive Team are big fans of. And, you know what, it wouldn't hurt for us at TV Tropes to use that feature more often too. You know, for putting up copies of important videos and archives of webcomics and such.

And yes, it gets its name after the WABAC time machine from Peabody's Improbable History. And now you know.