Posterous has done well eliminating the login hurdle.
Has anyone been able to do this for a voting/rating site?
I've been thinking about this for my site BuyersVote.com. I let people do some actions (like start a new category) without logging in, but rating I haven't figured out how to avoid abuse without logins. Offering logins with OpenID (and rpxnow.com) reduces the hurdle significantly since people don't have to create a new account, they can use an existing one, but it's a hurdle none the less. Thanks for any tips!
...no you can't make a credible voting or rating system without a login.
It's too easy to game the system to create repeat votes via cookie burning, ip rotation, etc (see the other responses).
Forcing someone to be logged in and then only allowing one vote per poll helps keep all but the most persistent griefers.
To that end, be aware account creation can be automated and unlimited unique email addresses created from one domain.
However, the difference between gaming the system by creating lots of fake accounts and gaming the system via cookie destroying/IP rotation is that the former is easy noticed within your system - assuming you have built such checks in place. It's also lets you easily remove the offending over-votes to restore calm to your survey/poll/etc.