The Open Gaming License
I stumbled upon the following text on WOTC's website here:

Q: Why create Open Games?

A: The tabletop RPG business lost 60% to 70% of its unit sales from the period from 1993 to 1997. After a detailed study of the market data available, business managers at Wizards of the Coast decided that the primary reason for this decline was the dissatisfaction consumers had about the products game publishers made available for sale.

One way to help publishers make products that will be more interesting to consumers is to allow them to use standardized systems that have large networks of players. Designing a product targeted at a large network of players gives that product a better chance of being commercially successful than designing a product targeted at a small, or a new network of players.

Due to the history of copyright litigation, and the relatively modest financial resources of most game companies, an informal agreement regarding the use of shared game rules, or a license requiring a monetary payment to a third party would not have been sufficient to generate sustained interest in using such systems. Open Games provide both a royalty free license (meaning they impose no financial burden on the publishers) and a formal, explicit agreement describing how to use copyrighted material owned by others without triggering lawsuits or threats of litigation.

Q: Any other reasons?

A: Yes. In addition to the potential improvement in the business of game publishing, Open Games will be subjected to a large, distributed effort to improve the games themselves. Because Open Game licenses allow publishers to make any changes they deem necessary to the material they are using, a publisher who thinks they have found a better way to write a game rule will be free to do so. And, if that new way is perceived as better than the existing alternatives, other publishers will be able to take that new rule and use it as well. In this way, the overall design of an Open Game should improve over time, and be the benefit of far more development and testing than any one game publisher, no matter how large and successful, could hope to apply by themselves.

It's interesting to note that bearing the d20 license has some restrictions, but one CAN publish their own set of character creation rules and whatnot if you don't stick the d20 license. Some creations sport the OGL logo. Interestingly the game system Action! uses the OGL, but doesn't use d20 at all. It just uses the license... protecting the "open" rules but protecting all the copyrights and trademarks of the intellectual property in the setting... and some rules...