NetBSD - writing device drivers
 
http://www.home.unix-ag.org/bmeurer/NetBSD/howto-lkm.html
http://man.netbsd.se/?find=driver+9+162 
http://cvs.gcu.info/cgi-bin/cvsweb.cgi/src/sys/lkm/misc/example/?cvsroot=NetBSD
http://cvs.gcu.info/cgi-bin/cvsweb.cgi/src/sys/net/if_tap.c?rev=1.7;cvsroot=NetBSD
http://netbsd.org/Documentation/kernel/porting-freebsd-net.html