that's almost every time illegal
but...some apps people use are gift (for fasttrack = kazaa and for openft), it's frontends like toxic and apollon, for DC there is Valknut (that is actually dcgui-qt) and then there are all these mutellas and gnutellas and stuff....really much. I'd say Valknut (= dcgui-qt) and gift + toxic (for Gnome) or apollon (for KDE) are the nicest..and for gift, fasttrack, openft and perhaps gnutella plugins