Following the release of the d20 SRD by Wizards of the Coast, numerous third-party publishers decided to hop on the bandwagon. This represented an explosion of creativity as well as, by the predictions of Sturgeon's Law, a metric assload of crap.
Eventually, the field begin to winnow. Some publishers could not meet the grade. Some could not make a profit. A few ran aground of the terms of the d20 System license and the OGL. Eventually, the market achieved some level of balance.
The official end of the era came with the announcement of Dungeons & Dragons Fourth Edition. The succeessor to the d20 System license, the GSL, embraced an entirely different approach to enlisting third party publishers.