- Pre-OGL: Before 1999
- OGL/d20 Era: 1999-2007
- Boom: late 2000 to mid 2002
- Bust: 2005 to 2007
- Post d20
To Understand the Bust, you must understand the Boom
When the Open Gaming License (OGL) was used on D&D 3.0 in 1999, along with the D20 System Trademark License (d20 STL), it ushered in a new era of 3rd party licensed supplements.This is called the d20 era, because of being linked to the d20 STL.
Many were acceptable, a number were excellent... and a lot were utter drekh.
The Boom was the roughly late 2000 to mid 2002 high point of anything with a d20 System Logo selling. It allowed many existing publishers to now support D&D, which had been the major game in the industry. It allowed many small publishers to launch with materials for the number one game in the world. It made licensed games easily published.
The d20 STL and OGL in combination resulted in almost no barriers to small press publishing. Moreover, many companies with strong but 3rd tier sales were able to put out d20 versions of their setting materials in a hurry. But anyone with $1500 and a decent computer could produce a short run, and get product into the pipeline. There was little review, be it professional or fan based, and purchasing agents for stores had a hard time sorting the good from the bad.
This was neither sustainable nor terribly good for the core engine. By 2005, the market was collapsing from too many low quality supplements.
The d20 Bust
The wide variety of low quality supplements resulted in many stores having surpluses of lousy supplements; for many, this was a major hit to their profits, and often, had the effect of damaging all but special orders for 3rd party supplements.
By 2005, the d20 Logo was a liability for new publishers, rather than an asset. It was also turning into an albatross around D&D's neck. Certain major lines of 3rd party product were selling well, most of which were written such that, while adhering to the letter of the d20 STL, you could easily use them without the D&D 3E or 3.5E Player's Handbooks. Further, most of these lines produced a corebook, and then, in a non-d20 branded "web product" included the material the d20 STL required being left out.
Many of the hundreds of publishers from the Boom period dried up and disappeared. Many of the pre-OGL companies also got hurt in the process. And, because many third party publishers had developed followings, they were able to launch d20 derived materials... materials like True20, Mutants and Masterminds, Prime Directive Modern, Spycraft, and T20 Traveller's Handbook. [Mongoose's] Conan, Materials from companies with known quality, and using the bulk of the d20 rules, but different enough to be unique games. People had to relearn less, and so these games diluted the market for D&D.
Also, in the same time frame, the Old School Renaissance OSR arose. While not actually part of the d20 boom nor the d20 bust, it was part of why there was...
D&D 4E and the End of the d20 STL
When the 4th edition of D&D was released, HasBro/Wizards decided to no longer release the core rules as open content... They discontinued the d20 System Trademark License. They couldn't discontinue the OGL, but they didn't have to use it themselves.
This is the start of the Post-d20 Era.
Paizo, who had been publishing Dragon and Dungeon under license, took the d20 System Reference Document, made a few tweaks, and announced their new engine: Pathfinder. In all ways that matter to gamers, it's still a d20 system game. Legally, however, it isn't - it has no compatibility indicator with D&D 3E/3.5E, and doesn't use the restricted trademarks made available under the d20 STL.
Many other d20 games came out with new editions that lacked the d20 logo, and included the "missing" text. Others simply stickered over the d20 logo and license.
The OSR games flourished, especially since D&D 4E was seen as a major rupture with what little continuity d20 had given to prior editions. They couldn't use the name D&D, but many are very close variations on older editions, including variations on Original D&D, AD&D, Moldvay/Cook BX, Mentzer BECMI, and even AD&D 2E.
The OGL in the post-d20 era also enables a number of second string companies to get initial bursts of new material. Mongoose, especially, has made use of this for their licensed non-d20 Traveller and RuneQuest games. (RQ has since expired, but they keep the rules in print under a new, non-licensed trademark.)