Anatol Romanov (5/9/2008)
All extended stored procedures mentioned in the article are well known to most of the people working with SQL Server for some years. They have been discussed many times in SQL server books and on popular SQL Server websites. Google search on "undocumented sql server stored procedures" returned 89,000 web pages. xp_FileExists for example is discussed at sqlservercentral, developersdex, databasejournal, sqlteam, dbforums, etc. The article lacks novelty but may be useful for some community members. That's why I rated it as 3-star.
You are absolutely right that these are all fairly well known amoungst certain communities of users, and I am certain that sp_who2 in particular is well known by most people who read this site. I certainly did not expect it to be breaking much new ground. Alexander Chigrik for instance has an excellent article on this same site on SQL Server 2000 undocumented procedures.
My primary motivation in writing this article was for my own team, who I found out in conversation was not aware of all of these. I went looking for a list that I could give to them, but I never found a single article that I was happy with for that purpose. Many were focused on SQL Server 2000, and there were some changes between 2000 and 2005. Others were too comprehensive and would have required my people to wade through lists of procedures which dealt primarily with replication which we do not currently use. Others only dealt with one of them instead of providing a complete list I wanted.
In the end I spent some time on a Saturday writing a list specifically for them that I thought would be useful to them. Then I decided there might be others who would find my particular list useful so I cleaned it up and submitted. Hopefully it will be of interest to others on this site.