PD2 is a mod for Diablo II: Lord of Destruction which includes many quality-of-life and balance changes, along with some new content. It aims to maintain the D2:LoD experience while improving on the game as if development never ceased.
PD2 offers increased build diversity as well as many other improvements over the vanilla game. See General Changes for an overview of improvements.
PD2 is free, but a legit copy of D2:LoD is required. You can purchase it from Blizzard Entertainment. English Version only.