Currently there are (in my mind at least) four good spam protection plugins for wordpress and wordpress mu. Each one has their merits and I have used/am using all of them across sites.
First is Akismet. Akismet is created by Automattic, the team behind the creation of wordpress.org, wordpress.com, buddypress.org, and bbpress.org. All projects listed above are opensource except for wordpress.com and akismet. Akismet is probably the number 1 spam blocking tool for wordpress blogs and is truly amazing. It has just 1 problem, it requires an API key to use. So no problem, just go to www.wordpress.com and get one, right? Well yes, if you are running a small enough number single user blogs. The problem happens when you try to create a blog network or use a WPMU installation. You could hardcode your API Key into your plugin file but then you will get banned, very soon. Instead you need to purchase on of the Akismet Business or Organization Keys. Those are expensive and as I am using multiple sites with different wordpress and wordpress mu, I chose to use Akismet only on www.biologypowerpoints.com because it is a single site with only 1 blog installed.
Next is reCAPTCHA! In my informal testing on http://wpmu.zyrot.com it appeared that reCAPTCHA caught 99.9% of the spam. It works well as a spam prevention tool however the fact that it uses captchas is a really good reason to choose another spam protection tool. It also requires an API Key to function with however the API Key is available for free from http://recaptcha.net/. If you prefer a captcha based approach, reCAPTCHA is the tool for you.
After reCAPTCHA comes one of my favorite tools for Spam Protection, Bad Behavior. Bad Behavior seems to be doing a reasonably good job blocking spam on this wordpress mu site. It also requires an API Key which is free to get from it’s own website. It has worked well at blocking spam for me however it seems to be blocking quite a few legitimate users as well. It appears for my needs to be working but I may need to reconsider at some time.
Finally, the last plugin that makes this list is WP-SpamFree. Out of all the plugins on this list, WP-SpamFree is the only one that doesn’t require an API Key to function. This means that you just install the plugin and it works. WP-SpamFree has the most user-changeable options and has an extensive help system built into the plugin. For standard WordPress (single blog) users it works perfectly outside the box. However for WordPress MU users, it unfortunately allows the administrator of any blogs the options of the plugin. Most site owners don’t want this and as such I have modified WP-SpamFree so that it only uses the options that the site admin sets as the global options and that the options configuration page is invisible to all users except for the site admin. I’m waiting on a response from WEBGEEK before I officially release as a modified plugin.
To anyone with a Standard WordPress blog, I would recommend WP-SpamFree as the Spam Deterrent off Choice and for those using WordPress MU, I would recommend WP-SpamFree MU (will be released soon)!