There are quite a few of these, and of course I'm not going to list all of them--Just the ones that, in my experience, do the best job.

Remember, while you will probably end up using more than one security module, try to avoid duplicating functionality. For example, there is little point in installing Akismet alongside Mollom, or the Flag module alongside the Spam module. Modules that try to take over the same functions are more likely to conflict with one another, and even if they do not conflict, every module you have enabled puts a little more of a load on the server, and makes your site run just a little slower.

And, as always, RTFM before installing anything.

Best in show
Mollom
Mollom is both a module and a service by Dries Buytaert, the original developer of Drupal, who still oversees Drupal development. It helps block fake registrations, spam posts, etc., with an average efficiency of 99.96 percent. In my opinion, Mollom wins hands-down over any other security module, or any combination of other security modules. It's the main security module in use here, in fact.

There is a free version for sites with fewer than 100 posts per day, and a couple of tiers of paid versions for those with more. Now, while I have no official data from Mollom on this, if you're running a charitable non-profit with more than a hundred posts a day, I'd email them and see if they're willing to work with you. They're pretty nice guys.

There are alternatives to the above, of course, and even Mollom will probably not fulfill every single one of your security needs, so here is a partial list of others, including a couple of alternatives to Mollom.

Runners up
Captcha
Ye olde familiar Captcha generator, with several options. When I've used this modules, I generally used the math captcha that comes with the basic package, but if you upload a true-type font to your server, you can also set up the traditional text-image Captcha. There is also a set off add-ons for Captcha-- CAPTCHA Pack
reCAPTCHA
A module that integrates Drupal with the free reCAPTCHA service.
Bad Behavior
Bad Behavior is a set of PHP scripts which prevents spambots from accessing your site by analyzing their actual HTTP requests and comparing them to profiles from known spambots.
Block anonymous links
If you allow anonymous comments on your site (I don't), this is a must-have.
Troll
Tools for dealing with trolls, including an easy way to ban by IP address. (Use that carefully.)
Quota by role
Useful if you start getting a lot of newly-authorized users spamming things up.
Spam
The spam module provides numerous tools to auto-detect and deal with spam content that is posted to your site. Spam can be automatically unpublished and/or deleted.
Akismet
This module is similar to Mollom, but, from the description on drupal.org... \"this module is pretty unsupported at this point. You might want to consider using Mollom.\" It does, however, work just fine.
Flag content
This module allows users with the proper permissions to flag nodes, comments, or users for the administrator to review. Examples are offensive content, spam, off-topic stuff, ...etc.
abuse
This module allows users to flag nodes and comments as offensive for the adminstrator to review. This is a lot like the flag module, but instead of having a separate link for each type of objectionable content (like the Flag module can do), there is one link for users to click, and then an opportunity for that user to add a comment to the submission to let the admin know why they flagged it.

The VidoopCAPTCHA module looks promising, but I haven 't actually tried it out yet. If anyone has, please post your experiences with it as a comment below.

For a more complete list of available Drupal 6.x security modules, register at Drupal.org and go to http://drupal.org/project/modules?filters=drupal_core:87%20tid:69&solrso...