Mixxx is free, open source DJ software which supports the Hercules DJ Console RMX (and the Hercules DJ Console RMX 2).
The Hercules DJ Console RMX is a USB MIDI controller with a built in
sound card. It is compatible with Mixxx versions 1.6.1+herc and later.
Both audio and MIDI works in Linux (from kernel ~2.6.27+), Windows
(XP, Vista, 7), and MAC OS X (10.4.11 (Tiger)/ 10.5.x (Leopard)/
10.6.x (Snow Leopard) 32-bit). It has been discontinued in favor of the
Hercules DJ Console RMX 2 which is USB MIDI class compliant and requires
no special driver.
As the controller is quite old, the page does not appear to have been updated for newer OSes. The manufacturer's driver page indicates continued compatibility with Windows 10 and OS X 10.10:
Drivers for MAC OS X and Windows can be found on the Hercules support
page. Same package for both Audio and MIDI.
And, if you are a linux user:
The audio device on the RMX is USB-audio class compliant and works
well with the ordinary ALSA USB-audio driver. This should work out of
the box on most distributions. [...] Hercules has released a common
MIDI-driver for their DJ controllers.
For further details on that MIDI-driver you should refer to Mixxx's Hercules Linux kernel module wiki page.