The NewDark patch emerged in late 2012 and contains many updates which make running Thief 1, Thief Gold and Thief 2 much easier on modern hardware. For convenience, we provide links to the discussion and utility threads which can be found here on TTLG. Although we have not heard of serious problems using the patch it must be stressed that the patch is neither endorsed nor supported by TTLG itself and its use is entirely at your own risk.

Patch installer for Thief 1 / Thief Gold:TFix v1.23 - (Note - TFix is currently using NewDark v1.24)
Patch installer for Thief 2: Tafferpatcher v. 2.0.18 - (Note - Tafferpatcher is currently using NewDark v1.25)

Discussion for latest version can be found here: Version 1.25
Thief 2 v1.25 : Mirror | Taffer's Paradise | Southquarter

Older versions (download links within thread):
v1.19 Announcement and Discussion thread
v1.20 Announcement and Discussion thread
v1.21 Announcement and Discussion thread
v1.22 Announcement and Discussion thread
v1.23 Announcement and Discussion thread
v1.24 Announcement and Discussion thread