How should I cite an archived webpage? While researching for an article, I came across an interesting post forum post which I wanted to use as a reference.

Before I could finish my article, the forum site I wanted to reference was officially shut down, making the aforementioned forum post inaccessible.

Thankfully I was able to find an archived version of the post on The Wayback Machine, but I'm not exactly sure how to reference it.

How should I cite an archived version of a webpage if the original webpage is no longer available? Should I add a "Date Accessed" note to my original citation, or should I cite the archived version instead?
I would consider doing both - the original (usually with a date accessed anyway these days - although I don't do much website citing), and a note that it is still available (as of your article) on the Wayback machine at the given address. 

Now, it is always (sadly) possible that Steam would consider requesting the Wayback folks to delete their listing in the future, but it provides a hint. 
Further, I'm surprised that Steam did not migrate the old site over to their new site, but that might have been a lot of work.