This question comes up often enough so it seemed to make sense to make a list. Here are the DACs that according to their manufacturer handle 24 bit/192kHz via USB input. I'm sure I missed some so please feel free to add your list in the comments. Note: for those companies with more than one 24/192 USB DAC I've only listed one, typically the least expensive.
Abbingdon Music Research (AMR) DP-777 ($4,995)
Acoustic Plan DigiMaster ($4,995)
Aesthetix Pandora ($6,000)
Antelope Audio Zodiac ($1,895)
Apogee Electronics Duet 2 ($595)
April Music Eximus DP-1 Ultimate 24/192 DAC-Preamplifier ($2,295)
Audio Aero La Fontaine ($19,000)
Audio Research DAC 8 ($4,995)
AURALiC ARK MX+ ($1,499)
Ayre QB-9 ($2,750)
Bladelius Design Group USB DAC (€ 550)
B.M.C. Audio DAC1 (HR) ($5,690)
Calyx DAC 24/192 - Digital to Analog Converter ($1,500)
Cary Audio CD 303T SACD ($6,500)
dCS Debussy DAC ($10,999)
DIY HiFi Supply Cleo Max 24bit 192khz USB DAC with Tube Output Stage ($1,115)
Docet U-4DAC ($750)
EAR 192 DACute ($5,795 black)
Eastern ElectricTube DAC +Plus ($1,100)
Empirical Audio Overdrive® USB DAC/Pre ($3,999)
E-MU 0204 USB 2.0 ($149.99)
Esoteric K-03 ($13,000)
Firestone Audio ILTW 24 bit 192kHz Asynchronous DAC ($367)
Grace Design m903 ($1,895)
KingRex UD384 (€420)
Light Harmonic The DaVinci DAC ($11,999)
Lindemann USB-DAC 24 bit/192 kHz ($990)
Lynx Studio Technology Hilo Reference A/D D/A Converter System ($2,495)
M2TECH Young 384/32 D/A converter ($1,799)
Meitner MA-1 Stereo D/A Converter ($7,000)
MSB Technologies Power DAC ($3,995)
Mytek Stereo192-DSD-DAC ($1,695 Pre version)
NAD Electronics NAD M51 Direct Digital DAC ($1,999.95)
North Star Design USB dac32 (€ 1,800)
Onkyo P-3000R ($1,699)
Peachtree Audio Grand Pre ($2,999 available December 2011)
Pioneer N-50 ($699)
Playback Designs MPD-3 ($6,500)
PS Audio PerfectWave DAC MK.II ($3,995)
Red Wine Audio Isabellina Pro ($2,500)
Resolution Audio Cantata Music Center ($6,000)
Resonessence Labs Invicta ($3,995)
RME Babyface ($749)
Schiit BIFROST DAC ($449)
Simaudio Moon Evolution Series 650D DAC ($9,000)
TAD TAD-C2000 ($24,500)
Wadia 121Decoding Computer (Projected price: $1,299)
Wavelength Audio Crimson HS Directly Heated Triode all tube USB DAC ($7,500)
Wyred 4 Sound DAC2 ($1,499)