I rely heavily on d6 ability checks in my games. I like them because I can adjust the number of dice rolled to reflect the difficulty of a check, yet the rolls still maintain a bell-curved probability (SEE HERE for my previous description of this approach).
To be honest with you, I don't recall where I first heard about this method, but I think I initially read about it in an online forum a long time ago. At conventions I've seen a few other people use the method, but it doesn't seem to be common. Chris Kutalik mentioned that Rob Kuntz uses this method in his games, which got me to wondering how long this method has been around.
Well, now I know the answer is at least 34 years. I was browsing through some issues of the 70s U.K. fanzine Underworld Oracle the other night and I noticed in the first issue, from 1977, that there is a 3d6 Intelligence check described. Specifically, it is called for as a defense against a PC viewing an Apparition. I also recall seeing similar checks in later issues of UO as well.
Great! So d6 ability checks officially get the Old School Stamp of Approval and I won't have to worry about tearing up my OSR membership card anymore. Whew...
Here's the evidence: