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: 
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.
stackoverflow has a few features that help with this; I think the single most useful step you can take is disabling the ability of anonymous users and new accounts to vote. This way, no one can sign up for hundreds of accounts and use their one vote to overpower other users. I'd say requiring a few posts or membership for a certain period of time are both decent options.

Some would say you could allow one vote per IP address to help address this, but I've played plenty of games where malicious users with a nigh-infinite number of proxies defied IP address-based security. It's a deterrent, but a savvy user will get around it easily.