WP-Spamfree is a fantastic free Wordpress plugin that keeps you from getting comment spam from automatic programs (common ones are for male enhancement drugs, online gambling, etc.).
You can get WP-Spamfree here: WP-Spamfree
The simple fact is that as your wordpress blog grows more popular, it will eventually be found by automated bots that will insert spam comments with links to their sites. Their intent isn’t so much to get people to click on their spam links, but rather to increase their search engine ranking by the number of links they’re getting to their site. I’ve literally had hundreds of comment spam links appear on my site overnight.
Let me tell you, deleting those spam comments is a pain. Wordpress gives you some built-in moderation filters to try and prevent it, but alas these guys are used to that and good at getting around them. WP-Spamfree uses some neat tricks to verify if the comment is being made by a person or a robot (using cookies, among other things).
The best part of WP-Spamfree is that it just plain works. There’s no text that the user has to fill in or anything — making a comment is just as simple and easy as if there was no filter at all. It doesn’t put comments in a moderation queue that you have to manually check, and I have never seen a false positive from it. It just detects robots and doesn’t allow their spam comments through. It even includes a little tracker to show you how many spam comments it’s caught. Here’s a shot of the tracker from one of my blogs at the time of this writing:
I have a blog with over 5,000 visits a day. It gets a lot of traffic and used to get a lot of spambots. Since installing WP-Spamfree I haven’t gotten a single automated spam comment. I not only strongly recommend this free Wordpress addon, but I consider it the only addon that is essential for any blog.
Now keeping trolls out of your comments is another matter 
