What are some alternatives?
When comparing Mixxx and Qtractor, you can also consider the following products
Mixmeister - Create custom party mixes, burn CDs, or add special effects to MP3s with this line of DJ software for the novice and pro. Offers free trials and user community.
Ardour - Record, edit, and mix on Linux, Mac OS X, and Windows.
Cross DJ - Mix, scratch, sample, record: rock great parties with top-DJ features.
FL Studio - Image-Line's FL Studio, now on it's 12th version, is a well-known music production suite and the most popular beat processor on the market, due no doubt to its longevity. Read more about FL Studio.
DJ Mixer Pro - DJ Mixer Professional is the best DJ mixing software for Mac and Windows, Support audio, karaoke and video mixing, automatic DJ mixing, one-click beat-matching, etc.
LMMS - Make music with a free, cross-platform tool