ShanDj
A very simple HTML5/Javascript DJing software for local use
... 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.