ShanDj
A very simple HTML5/Javascript DJing software for local use
...You'll have to download it, place the ShanDj.html file in the directory where the audio files are, then open it in your web browser. Audio formats supported are varying depending on the browser. HTML5 browser support and Javascript required (browsers newer than 2008). Features simple playlists and a crossfader to mix between two currently played tracks, and a RobotDJ mode.
If you are looking for a similar tool, but for use over internet instead of locally, you should take a look at Slcnc CMS, from the same developer, which integrates DJing over internet among other features.