VirtualDJ

It is used by DJs to replace their turntables and CD players, and use digital music instead of vinyl and CDs.

In the same way that the CD players used by DJs have more options than a regular Hi-Fi CD player, VirtualDJ has more options than a simple media player like iTunes.

It lets you "mix" your songs, by playing two or more tracks at the same time, adjust their relative speed so that their tempo will match, apply effects like loops etc, and crossfade from one side to the other.

It also lets you scratch your songs, set and recall cues, and all the other regular features DJs expect to find to be able to mix.