Mixxx is one of the best free and open source music making applications: not only can you download it for nothing, but the more adventurous can also delve into the code and adapt its feature set.
As standard, Mixxx features twin decks (each with its own scratchable, scrolling waveform) iTunes integration, four sample decks, loops and hot cues and support for a wide range of track formats. There's BPM detection and sync, and also an Auto DJ function for when you're feeling lazy. There's support for more than 30 controllers and timecode vinyl systems.
If you don't currently have a favourite DJing app, Mixxx has got to be worth a try. You have, after all, got nothing to lose.
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