What I've done:

The arp.c file sends 2 ARP requests, one to the default gateway and one to the
victim, to get their MAC addresses. I then use these addresses to construct a
phony ARP response to the victim that tells them that I am their default
gateway. Some operating systems (e.g. Linux) are very suspicious of spurious ARP
responses like this and will quickly figure out the ruse, but Windows will
happily chug along in blissful ignorance until you annoy it by taking too long
to respond to requests. To prevent even this weak defense mechanism from
deploying, the program has an option to keep resending the phony packet every X
number of seconds. Note: if it takes more than about 1 second to print the
second line you probably got a response from the wrong person, so try again.

Syntax:

./arp VICTIM_IP [ TIME_INTERVAL (in seconds) ] [ -q (quiet mode) ]

Another note: setting the time interval to 0 means only one packet is sent

The sweep.sh file gets the current IP address and routing prefix in CIDR
notation and from this calculates the netmask, then uses this information to
ping sweep everyone on the current subnet and attempt to send phony ARP packets
to them using the arp.c program. Obviously you shouldn't do this unless you know
exactly who will be affected by it and you're prepared to handle the
consequences. There is also a flag to do only the ping sweep and not attempt the
spoofing, which prints out the IP addresses of everyone who responded in some
fashion to the ping from the tcping program (not mine) on port 6000. For some
reason I was unable to determine, Windows responds to random TCP connections on
port 6000 but firewalls almost everything else, including ICMP. Go figure.

Syntax:

./sweep.sh INTERFACE_NAME [ -p (ping only) ]

cage.sh is a simple URL rewriter script for Squid--it just takes any URL and
turns it into a picture of Nicolas Cage.

There are also some other relevant things, for example the sample website is
available under the http directory, and the Apache and Squid configuration files
are here as well. And one more thing that doesn't really fit neatly anywhere
else: the iptables rule to redirect the traffic. By default Linux just forwards
any IP packets destined for elsewhere, so the ARP spoof by itself won't really
affect anything other than making the victim's packets pass through your
machine. However, since it's *your* machine, you can have your way with them, so
we set up iptables to redirect anything coming through port 80 to port 3129 on
our machine, which Apache is listening on as well as Squid (just make sure
you've set them up not to conflict if you're running both). Note that HTTPS
(port 443) is significantly harder to mess with, so I don't try.

iptables rule (run as root):

iptables -t nat -A PREROUTING -p tcp --dport 80 -i $INTERFACE_NAME -j DNAT \
--to $DEST_IP:$DEST_PORT

Things you change yourself are prefixed with $'s this time to distinguish from
literal ALLCAPS arguments.

P.S. if you didn't notice, there's a Makefile to build arp.c with the correct
options.