Because many of you have asked for it, we've added the option to allow anonymous votes to your coRank site.
Now, if you don't want people to register in order to vote at your site, you can go to your admin pages, tab "Advanced", where you'll now find the option:
Allow anonymous votes?
( ) Yes, unregistered users can vote
( ) No, users must be logged into their account
Of course, you must have voting enabled in order to see this new option.
How does coRank know someone has already voted to avoid having one person to vote several times on the same story? There really are two non-rocket-science way to do this: by registering the IP, by using a cookie, or both. We've opted for the IP method so far, but of course this is subject to change. The IP method by itself has the disadvantage that it may create false positives for people connecting through a proxy as well as dynamic IPs that change hands every now and then, but as I just said, this is merely a first implementation that will become more robust over time.
Note that if you visit a coRank site that allows anonoymous votes, you cannot just vote anonymously while signed into you account. You'll need to sign out, then vote.
In my opinion, the advantage of allowing anoymous votes is that you can get more people to participate and vote, and that's a good thing. The drawback is that your site won't get as many registered users, and those users who don't register will not experience all the good stuff they get from registering. We believe however that if you allow users to vote without registering, eventually those who like the service will end up registering anyway. Either way, it's up to you!
FYI, at this point our original coRank site does not allow anonymous voting, but we are considering enabling it as well.