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 / 6 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: about 1 year 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: about 1 year 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

I love all the tools from PlaylistMachinery when I'm looking to do things Spotify won't... Like dedupe. http://playlistmachinery.com/. Source: about 1 year ago

There's no way to do it inside the official Spotify app. There are some 3rd party tools that offer some interesting functionality though like http://playlistmachinery.com/. Source: over 2 years ago