"Dan suggested that, in the interest of helping me get up to speed with password cracking, I start with one particular easy-to-use forum and that I begin with "unsalted" MD5-hashed passwords, which are straightforward to crack. And then he left me to my own devices. I picked a 15,000-password file called MD5.txt, downloaded it, and moved on to picking a password cracker."

My guess is that the forum he mentions was the insidepro.com web site, since it is a popular password hash sharing site. A quick search there found several attached files named MD5.txt, but none seemed to be the right size or have the right hashes. However, a search for hashes mentioned in the article found this file (http://forum.insidepro.com/download.php?id=12783&sid=160...), which contains 16,880 (instead of the 16,449 hashes listed in the Ars article) and at least 3 of the MD5 hashes specifically mentioned in the article.

There's a good chance this is the same file or close to it, but you'd have to try matching it with more of the hashes from the article to know for sure.