Say what you will about the Third Edition of Dungeons & Dragons itself -- and I have -- but I cannot countenance anyone complaining about the Open Game License, which is nothing sort of a godsend for the old school community. Why, you might ask? Quite simply, the OGL literally ensures that the Gygaxo-Arneson heritage of the game is available to any and all in perpetuity. Without it, games like OSRIC, Labyrinth Lord, and Swords & Wizardy, among others, would not be possible. The OGL is nothing short than an insurance policy against the day when Hasbro decides to treat D&D like so many other bits of IP it controls and locks it away in a vault somewhere, never to be seen again (like so many Avalon Hill titles). One day, "Dungeons & Dragons," as a brand name may disappear forever, but the specific ideas and concepts of its creators are now forever available to the hobby to use, develop, and disseminate.
To cite some rather specific examples, let's take a look at some demons and devils created specifically for D&D and see how many of them are now Open Game Content:
Baphomet: This name is, of course, public domain, but the specific association of it with the demon lord of minotaurs is a Gygaxian invention. Thanks to the Tome of Horrors, you can now use him in your retro-clone products if you so desire.
Fraz-Urb'Luu: Famous for having been imprisoned beneath Castle Greyhawk, the demon prince of illusion is OGC, thanks again to the Tome of Horrors.
Geryon: Derived form Greek mythology by way of Dante's Inferno, D&D's serpentine ruler of the fifth layer of Hell is OGC.
The "Faceless Lord:" No, we can't call him Juiblex, but we can call him Jubilex. Again, ToH is to be thanked.
Kostchtchie: Again, a real world mythological name, but his portrayal as the demon lord of frost giants is unique to D&D -- and he's now OGC.
Moloch: Of Biblical origin, Moloch is the ruler of the sixth layer of Hell and lieutenant to Baalzebul. Both and the name of his master are OGC.
Orcus: Orcus has a real world analog, but the image of him as a bat-winged, goat-headed prince of the undead is pure D&D; it's also OGC.
Pazuzu: An ancient Baylonian demon famous in the 20th century for his connection to The Exorcist, D&D's version of him is now OGC.
Careful examination of other OGC products will reveal many other formerly D&D-specific monsters are now Open Game Content that may be freely used by anyone under the terms of the Open Game License. I find it hard not to be very happy about this, because it means, for example, that, were I to publish an old school gaming product, I am free to use many distinct elements of AD&D rather than having to create my own substitutes.
This may seem like a small thing, but it isn't, for much -- though not all -- of the appeal of old school gaming is its unique storehouse of monsters. Without them, there's a sense in which a fantasy game could never claim to be Dungeons & Dragons anymore. I know that, for many people, one of the reasons that 4e does not feel like a true successor to OD&D is because of the way it has abandoned or extensively reworked so many monsters that have been strongly associated with the game for 25 or more years. But, of course, if one's goal is to build an IP you can exploit and others cannot, you have no choice but to abandon the past, particularly since so much of D&D's patrimony has been bequeathed to us, the gaming community, to do with as we please.
And that's why I shall be forever grateful to Ryan Dancey and the Open Game License.