This patch sets the brightness to maximum and also enables a features menu.
For some reason the US release of Doom 64 had it's brightness set to zero.

Press PAUSE during gameplay to access the menu, which contains level select, 
invulnerability, health, weapon toggles and an option to view the whole map.

Update:
- Removed debug commands from the control pad options
(thanks to caterpillar for pointing it out)

- Now has patches for the Rev A, US 1.0 & PAL version

Doom 64 (USA) (Rev A).z64 (US 1.1):
  CRC-32: 1d3a17b5
     MD5: 1b1378bb9ee819f740550f566745af73
   SHA-1: 6fb0ce9c75bbe54b6e1ede337652b0221e5f2aad

Doom 64 (USA).z64 (US 1.0):
  CRC-32: 5cc1ade6
     MD5: b67748b64a2cc7efd2f3ad4504561e0e
   SHA-1: 799a588d73da3fcce8031026a8187da92b91c817

Doom 64 (Europe).z64 (PAL):
  CRC-32: d985c356
     MD5: 190ccca6526dc4a1f611e3b40f5131c0
   SHA-1: b63060f69bb4e1547da1d762e740d19393977055

Advanced:
Offsets for Rev A (US 1.1):
The brightness value itself is set at offset 5B47B
64 is max, 32 half (PAL) & 48 optimal (my opinion)
The features menu is at 5B483,  00 is off 01 on

Offsets for for Rev 0 (US 1.0):
The brightness value itself is set at offset 5B3CB
64 is max, 32 half (PAL) & 48 optimal (my opinion)
The features menu is at 5B3D3,  00 is off 01 on

Offsets for for PAL/EUROPE:
The brightness value itself is set at offset 5B61B
64 is max, 32 half (PAL) & 48 optimal (my opinion)
The features menu is at 5B623,  00 is off 01 on

Note: Editing this by hand will require a crc fix