A web enhancement was a supplementary addition to a Dungeons & Dragons book, published online as a free downloadable PDF in order to promote the book's release. Web enhancements often included material which had been cut from the original book for space considerations.
Web enhancements were limited to Dungeons & Dragons third edition (2000 - 2007). Although Wizards of the Coast would go on to offer web publications for other editions of the game, these did not use the web enhancement format.
D&D 3rd edition
93 separate web enhancements were released covering 77 books, beginning with a web supplement for the adventure module Deep Horizon (2001) on November 10 2001, and ending with Tome of Battle: The Book of Nine Swords (2006) on January 10 2007.
Dungeons & Dragons 4th and 5th edition did not make use of the web enhancement format.
D&D 4th edition discontinued free web content in favor of the D&D Insider, a paid offering with a monthly subscription fee.
D&D 5th edition did not use web enhancements, but did offer Unearthed Arcana, a series of free web releases of preview content intended for potential release in future products.