As the saying goes, a mastering engineer's most important tools are his ears and experience, but for those who are interested these are the tools involved:

ANALOG/DIGITAL CONVERSION:
Cranesong HEDD 192
Benchmark DAC-1

MONITORING:
Dangerous Music Monitor ST
Focal Twin 6BE + Sub6
Avantone Mix Cube Active
Sennheiser HD580 Headphones

ANALOG COMPRESSION:
API 2500
Manley Vari-Mu

ANALOG EQ:
Manley Massive Passive
API 5500
Siemens W295b (pair)


ANALOG MISC:
Rupert Neve Designs Portico 5014
Dangerous Music S&M

Siemens IA line amp (pair)
Crookwood Igloo VU meter
Rascal Audio Analogue ToneBuss
Otari MX5050 1/4" reel-to-reel

PLUG INS + DIGITAL:
Universal Audio UAD-2
Audioease
Brainworx
DDMF
Izotope RX Restoration
Sonnox
Stillwell Audio

Symbolic Sound Kyma
Eventide H7600