The main reason to use a Mac as a router is to provide
an IP over localtalk
to IP over ethernet gateway.  It's easier to find an ethernet card for a
Mac II than a localtalk card for a PC.
  I use a Mac IIci
 running NetBSD. Check out this web site for details, and email me if you
 need some network cards etc. cheap. 
Does NetBSD support IP over localtalk?  If so, I may switch over. 
I don't know about NetBSD, but I kind of doubt it. Its a Maybe via Apples
free LocalTalk Bridge software. The Shiva/Kinetics FastPath and some of the
Farallon boxes yes. My NetBSD router IIci is pure ethernet, and one of my
other normal macs does the LocalTalk Bridge, but I only use it for printers
right now. IP over localtalk is on my future agenda though.