https://help.ubuntu.com/community/UbuntuStudio/SupportedHardware
USB
Most stereo devices will work out of the box (unlike multichannel devices), but often not with settings above 48kHz/16bits. This is due to most devices not following the 2.0 usb class definition, while most devices still adhere to the usb 1.1 class definition. With usb1.1 the performance is much more limited, and therefore the number of channels, as well as the samplerate and bits range are limited. A few devices stand out, by either having device specific firmware available, or being usb 2.0 class compliant.
Note that the devices below do not make a complete list of fully supported usb devices by any way.
Well supported:
Presonus 1818VSL (supported with the newer versions of alsa) See http://lists.linuxaudio.org/pipermail/linux-audio-user/2012-May/084993.html
Focusrite Scarlett 2i2 - USB 2.0 Class complient and Fully supported out of the box with ALSA (snd-usb-audio module). Seehttp://wiki.linuxaudio.org/hw/focusrite_scarlett and http://focusritedevelopmentteam.wordpress.com/2012/04/23/linux-and-focusrite-novation-products/. Article confirming that this device is fully operational: https://community.ardour.org/node/4709. Specs available athttp://us.focusrite.com/usb-audio-interfaces/scarlett-2i2/specifications
Focusrite Scarlett 2i4 - USB 2.0 Class complient and Fully supported out of the box with ALSA (snd-usb-audio module). The 2i2 and 2i4 are both confirmed to work well with Linux, ALSA and JACK. Using the correct settings, you can achieve near 1ms latency with no xruns using 96Khz or 48Khz 24 bit audio with the Scarlett 2i4. See http://wiki.linuxaudio.org/hw/focusrite_scarlett Article confirming that this device is operational: http://www.linuxmusicians.com/viewtopic.php?f=6&t=10142.
M-Audio MTRACK - Class complient. This audio interface is based on the IO2 Express made by Alesis (they look identical in appearance and share identical specs). Specs available at http://m-audio.com/products/en_us/MTrack.html.
Alesis IO2 Express - Class complient. WORKS completely! Tested under Linux Mint 12. Seehttp://wiki.linuxaudio.org/wiki/hardware_support. Specs available at: http://www.alesis.com/io2express.
NI Komplete Audio 6 - Class complient. See http://wiki.linuxaudio.org/wiki/hardware_support.