I used to use soundsnap.com for a lot of things. Source: about 1 year ago

This project took me about 3 weeks of free time. I primarily used Studio One 5 pro for my DAW, and the East West Hollywood orchestra for most of the music. Foley recordings for the sound effects came from soundsnap.com. Source: about 2 years ago

In addition to the previously mentioned soundly, you can also try soundsnap.com. It has a subscription model that is unlimited downloads for a year, if I remember right. But that could be useful as a beginner as well. It doesn't have a fancy UI/library management tool like Soundly, but still worth looking at. Source: over 2 years ago

Mixxx is excellent and has pretty wide hardware support. FYI the "limited" functionality of Rekordbox can be hardware unlocked with some Pioneer controllers. https://mixxx.org/. - Source: Hacker News / 5 months ago

There are quite a few DJs that use Ableton for DJing (or hybrid sets). Of course, Ableton wasn't made with DJing as its main function in mind but you totally can use it for that purpose and (with some tweaking) do everything and more that would be possible in a dedicated DJing software (like Traktor or Rekordbox or Mixxx). Source: 12 months ago

I’ve used Mixxx a little bit and have been surprised how capable it is. It’s an open source project and is free to download: https://mixxx.org/. Source: 12 months ago

Mixxx (you already mentioned this; it's certainly a great piece of free software to test the DJ waters with and works with a lot of gear). Source: about 1 year ago

Alternatively, you can also use Mixxx with the S4 MK2. Mixxx is free. But instead of a Traktor control vinyl record, you'd use Serato timecode instead, as stated in the docs. Source: about 1 year ago