Mixxx is an open-source DJ application that allows you to mix your songs like a professional DJ. The application is free and it can be downloaded from the Mac App Store, where it ranks at number 25 amongst the most popular free apps. There are versions for Windows and Linux as well.
The application consists of two panels where two songs loaded at the same time. In between these two panels are the music playback controls, where you do all your DJing. At the bottom are all the songs that you load on your playlist along with all the more advanced playlists and the Auto DJ feature.
I will be honest about this. I don't know anything about DJ software. I am a novice user, if you will. But I can reckon that this app looks very professional. I can use it and make decent mixes. I can only imagine what someone with experience can do with this piece of software. I read quite a few reviews of this app on the Mac App Store, and not one of them was negative. Perhaps the only thing I would want is a bit of help for users like me, but I am not considering that a disadvantage at all.
All in all, this is great DJ software that is absolutely free. It is really easy to install and set up. Give it a try!