GemRB
About
Baldur's Gate, Icewind Dale, and Planescape: Torment are RPGs originally built on the Infinity Engine, the engine GemRB reimplements. GemRB is a portable, open-source reimplementation of the Infinity Engine with a cleaner, more extensible internal design than the original. It requires the user's own copy of the original game data to play a full game, though a small bundled trivial demo offers a limited preview, and it runs on a wide range of platforms including Linux, Windows, macOS, several BSDs, and assorted handheld/console/exotic systems.
How it differs from the original
GemRB doesn't bundle the original games' data itself — you must supply a legitimate copy of the game files (or use the included tiny demo) — and internally rewrites the original Infinity Engine with a different, more extensible architecture rather than reusing the original engine code.
Getting started
To play, install your own copy of the original game's data alongside GemRB — copied from a Windows installation, an official installer, via WINE, or extracted from the original CDs with the Unshield tool — or run the small bundled demo for a preview without owning the game.