The best open source alternative to Mixxx is Giada, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 25 alternatives to Mixxx and eight of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to Mixxx are Luppp (Free, Open Source), BpmDj (Free, Open Source), Internet DJ Console (Free, Open Source) and TerminatorX (Free, Open Source).
BpmDj is a program that can be used to DJ. It works under Linux and plays Mp3's/Ogg's. It has a fully automatic BPM counter. It can determine the sound color. It has a full fledged QT based interface. It will help you manage a large amount of songs.
Internet DJ Console is a project started in March 2005 to provide a powerful yet easy to use source-client for individuals interested in streaming live radio shows over the Internet using Shoutcast or Icecast servers.
TerminatorX is a realtime audio synthesizer. This software allows scratching audio data (wav,ogg,mp3, etc.) the way hiphop-DJs scratch on vinyl records. It features multiple turntables, realtime LADSPA effects, a sequencer and MIDI interface.
xwax is open-source vinyl emulation software for Linux. It allows DJs and turntablists to playback digital audio files (MP3, Ogg Vorbis, FLAC, AAC and more), controlled using a normal pair of turntables via timecoded vinyls.