In it you will find a wide assortment of important and less important fixes and additions making the games more playable and truer to their origins. For the details, try GemRB yourself or check the changelog below for some highlights.
Currently the sources are available and a windows build just a few revisions before the release. You can get both here.
Full changelog digest:
GemRB V0.6.5 (2011-07-24): New features: - quickspell support - keymap.ini handling - yet more unhardcoded PST projectiles - area comments, party npc interactions - ios port and enhanced touchscreen support Improved features: - performance - squirrels and other small game don't die instantly - stores and stacking - effects, triggers, actions, dialogs - secret door detection, turning undead - bugfixes and internal cleanups Applied patches: ios support patches from Brad Allred android and touchscreen patches from Beholder