Warhammer 40,000: Dawn of War

Patch the game to v1.2 or 1.3 (not 1.4+), then either
- pass in the w and h command line parameters like so
w40k.exe -w 1280 -h 768
- or edit the local.ini file and change screenwidth and screenheight

Patch 1.3 is provided here.

Patch 1.4 broke widescreen support: screen change is 4:3 pillarboxed. An older version of the game is required for Vert- gameplay.
Expansion Winter Assault, which requires the 1.4 update, has also no widescreen support.