Mixxx
Mixxx is a great piece of turntable and sound manipulation software. Filled with useful features and effects. Best of all, Mixxx is completely free and open-source.
Create playlists and use crates to plan your set and build a categories to organize your music. Search and sort to quickly find the track you are looking for, or sort by BPM to find your next track. Mixxx provides iTunes and Traktor Library Integration, drag'n'drop track from iTunes or Traktor into Mixxx.
Features include:
Four Decks
Pitch and Key Control
Beat Looping
EQ and Crossfader Control
iTunes and Traktor Library Integration
Free and open-source
Overall, Mixxx provides a feature packed, quality, piece of software that out-does plenty of other similar programs, for free. Bargain.