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: 
![alt text](https://i.stack.imgur.com/X11ol.png)

From [xkcd](http://xkcd.com/233/)
I'm a fan of the "hidden field" CAPTCHA. I don't remember where I read about it, but the idea is this:

* create your form as normal
* add an extra field but hide it (i.e. `style="display:none"` on the surrounding div or table row)
* after submission, if the field is blank, do the appropriate action (eg send an email); if the field has been filled in, then it's a robot submitter

The only case where this falls down is if the user's browser doesn't handle CSS (or they have it switched off), which is very rare.