The open-source GemRB engine is an open implementation of Bioware's Infinity Engine to handle running the game assets from Baldur's Gate, Baldur's Gate 2, Icewind Dale, and Planescape: Torment. GemRB is GPL-licensed and works on Windows, OS X, BSD, Android, iOS, and other platforms while being nearly feature-complete with the original closed-source engine. Various improvements over the original Infinity were also made like providing touch input support.
For those wanting more details about the current state of the game engine, see the TODO page that has completed and open action items for various Infinity Engine games.
The most recent release of GemRB happened in late November and is version 0.7.2. GemRB 0.7.2 features SDL2 software keyboard support, ToBEx flags for effect damage, output support for the in-game message window, confirmation when exiting, and saving configuration changes different. Improved features are listed as custom dialog.tlk encoding support, improved text, option windows, actions/triggers/effects/invisibility handling, and assorted bug-fixes.
Find more about this open-source game engine at GemRB.org.