How to use virtual sound hardware

[feanor <root.rlmh@gmail.com>]

I've read somewhere that you can use several virtual mixers with 
gnu/linux : my question is : is it possible ?
I mean : having something like /dev/dsp1; /dev/dsp2 with a software 
mixing it and giving this to the hardware... This would be quite usefull 
to me, when using skype and xmms...
Thank you
Romain