It is used by VTun (http://vtun.sourceforge.net).
Another interesting application using TUN/TAP is pipsecd
-(http://perso.enst.fr/~beyssac/pipsec/), an userspace IPSec
+(http://perso.enst.fr/~beyssac/pipsec/), a userspace IPSec
implementation that can use complete kernel routing (unlike FreeS/WAN).
3. How does Virtual network device actually work ?