Expanded Psionics Handbook
The Expanded Psionics Handbook (officially abbreviated as XPH for no clear reason beyond that the X makes it sound cool) is the Dungeons and Dragons 3.5e replacement for the Psionics Handbook. It flushes the psionic combat system down the shitter, replacing it with a system that's essentially just spells with power points instead of Vancian Casting instead of all the overcomplicated crap previous editions have used, adds a new base class (the Wilder), changes the Protoss Zealot from a prestige class to a base class, adds several new playable races (like the Dromite, Half-Giant, and Protoss), changes the stats for Thri-kreen. It also got rid of that mess known as psionic combat.
For whatever reason, most XPH content was released under OGL. XPH’s OGL content has the only mechanical change from closed content to OGL among WotC’s content, as the Slayer Prestige Class in the OGL release has substantially easier requirements than the Illithid Slayer found in the book. The two main theories of why are because it was referenced damn near everywhere, and because 3.5 psionics are heavily based on D20 Modern psionics and that system was already OGL. Whatever the case, this has been a massive blessing for psionics fans, as it enabled a wealth of third party content for the system.