Question: **What mechanisms do you know that prevent your site from being abused by anonymous spammers.**

For example, let's say that I have a site where people can vote something. But I *don't* want someone to spam something all the way to the top. So I found (a) creating an account and only allowed to vote once and (b) CAPTCHA to decrease spam. What other methods do you know and how good do they work?

Answer: 
![Captcha](https://i.stack.imgur.com/zyh7b.png)

From [Quantum Random Bit Generator Service](http://random.irb.hr/signup.php), via [MNeylon](http://www.mneylon.com/blog/archives/2008/09/02/captcha-sorts-out-the-nerds-from-the-boys/)
This is the study area of [Human Computation](http://en.wikipedia.org/wiki/Human_computation).

there is an excellent video from Luis von Ahn here:
<http://video.google.com/videoplay?docid=-8246463980976635143>