It's common knowledge that a mastering engineer's most important piece of gear are his ears. But for those who are interested:

ANALOG/DIGITAL CONVERSION:
Cranesong HEDD 192

MONITORING:
Dangerous Music Monitor ST
Focal Twin 6BE
Avantone Mix Cube Active
Adam A7
Sennheiser HD580 Headphones

ANALOG COMPRESSION:
API 2500
Manley Vari-Mu

ANALOG EQ:
A-Designs Hammer Tube Equalizer
API 5500
Siemens W295b (pair)


ANALOG MISC:
Rupert Neve Designs Portico 5014
Siemens IA line amp (pair)
Crookwood Igloo VU meter

PLUG INS + DIGITAL:
Audioease
Brainworx
DDMF
Flux
Massey
Sonnox
Stillwell Audio
TC Powercore
Universal Audio UAD
URS
Waves SSL


Symbolic Sound Kyma