anonymous voting, how do they do this ?

at wikio.com, you can vote anonymously, how do you think they are
doing this...?

- if I vote in FF, then clear cookies, it remembers I voted
- if I then vote in IE, it still remembers I voted...

so it must be server side, but checking source IP would not be very
good...

never mind, I just tried from my wifes machine, it still thinks I have
already voted, so must be source IP

The IP address is provided to you by your web server which gets it
from the TCP/IP connection between the client and the server. The
three-way handshake makes it impossible to spoof the IP address, so
this scheme works to ensure no dupes, but it causes problems if you
share a connection behind NAT. If someone on your shared connection
votes before you do, you're SOL.

In your rails controllers, you can get the IP via request.remote_ip.

CAPTCHAs, or requiring logins that are email verified... There are a
number of solutions; they all have their pros and cons and which one
is right for you depends on your app...