Here is a DJ Turntable app for our N900. It is a free sample project for QT developers, however it is fully functional and quite nice. More info and download here ([Only registered and activated users can see links]).

You must download the file named: turntable_v1_3_fremantle.deb

Screens:
[Only registered and activated users can see links] ([Only registered and activated users can see links])
[Only registered and activated users can see links] ([Only registered and activated users can see links])

Note: You can have your own tune playing. It has to be in a .Wav format though (uncompressed). You just click on settings in the app and navigate to the file.