Hercules Linux Kernel Module
Hercules released a linux driver for the MIDI part of their DJ consoles and controllers. Since it's been outdated by newer kernel versions, community members are now maintaining it, so it works also in newer kernel versions.
For Ubuntu 10.04 and up, you'll want lightrush's patched version https://sites.google.com/site/lightrush/random-1/herculesdjconsoleonkernel2635orubuntumaverick.
Some more (older) information can be found on these pages:
This is the Mixxx Wiki - 
Mixxx is a free and open-source DJ software.
Manual
Hardware Compatibility
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Coding Guidelines
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Developer Guide
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Contributing Mappings
Mixxx Controls
MIDI Scripting
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HID Scripting