Doug McCrae said:
Has much use been made of 3rd party produced OGC by other 3rd party publishers?
Yes.
Find a 3rd party product and read the Section 15 sometime. Complete rulebooks will tend to use more OGC than supplements, supplements will use more than settings, settings will use more than adventures.
For example, are there modules (either in print or pdf form) or websites making use of the Tome of Horrors monsters?
Is that a specific litmus test? I don't know the answer to that specific question.
I can answer from personal experience.
I developed my Vehicle Chase rules in Grim Tales from concepts in Spycraft. Barsoomcore (Corey Reid) took those rules from Grim Tales, worked them over, made them his bitch, and then republished the whole thing as
Hot Pursuit through Adamant Entertainment.
IIRC Hot Pursuit appears in the S.15 of True20, so that particular concept got refined through four different publishers and four different published works.
This is but one example of MANY.
And if not, of what value is 3rd party produced OGC?
No offense intended, and I am not singling you out by any means, but ignorance about the amount of 3PP OGC is not an indication that it doesn't exist.
(It's certainly a good indication of just how rarely folks flip to the S.15 designations, though for me it's Step 1 of any purchase.)