Atari Explorer was created in 1997 by Atari Historical Society co-founder Karl Morris, and ran until late 2007. An all-new redesign of the site is currently in the works, but until then I have mirrored this incredibly useful historical resource here with Karl's blessing. If you're into this kind of thing, I highly recommend you check out his excellent work in his new company Zafinn Books, where he writes all about - you guessed it! Atari history.
Having personally used this website for research over the years, it seemed such a shame for it to no longer be indexed and available on the wider web, so I took it upon myself to mirror it here in as complete a form as I could piece together. This mirror is based upon the Wayback Machine's 2007-06-25 14:27 crawl of atari-explorer.com, the last available before it got hijacked by spammers.
I have made no changes to the content or layout, only manually checking every page and fixing broken links and images where I find them - and adding this explanatory page. You can see the reconstruction and cleanup process in the GitHub repo here.
The sister site to this one, Curt Vendel's Atari Museum - is also available as a mirror here.