One that I have used and found very effective is Spam Karma 2. The onsite overview states:

Spam Karma 2 (SK2) is an anti-spam plugin for the WordPress blogging platform. It is meant to stop all forms of automated Blog spam effortlessly, while remaining as unobtrusive as possible to regular commenters.

Andy Roberts has an instructive commendation for Spam Karma 2 in a post from last August, Instant Karma for Comments and Trackbacks. Like Andy, I appreciate the fact that with Spam Karma 2 you can “see under the hood” as to what is happening, and you can tweak it. As far as I can see, with Akismet, what you get is all you see and you just have to trust (I may be missing something there, but if I am I don’t think it’s obvious – and I like obvious with this sort of tool).

Having monitored Spam Karma 2’s operation for a few weeks, Andy Roberts reported:

So far there have been no false positives and only 2 spams passed through out of thousands. Brilliant.

There’s also Bad Behavior, which I will comment on in another post.

Post from: EveryJoe

Spam Karma 2 May Be A More Helpful Comment Spam Blocker Than Akismet

A standard plugin for WordPress, the Automattic Kismet, or Akismet tool seems to me to operate very effectively as a spam catcher. I’ve been quite impressed, with this and otherr WordPress blogs of mine, that the various spam messages don’t seem to be getting through. And I have been quite pleased to hit the ‘delete all’ link to get rid of the spam items trapped by Akismet. 

The Akismet site describes the product rather romantically:

Automattic Kismet (Akismet for short) is a collaborative effort to make comment and trackback spam a non-issue and restore innocence to blogging, so you never have to worry about spam again.

I don’t know about “restoring innocence to blogging” but in the past few days I’ve realised I might have been somewhat innocent in trusting Akismet too readily, not so much in terms of blocking spam, but in terms of whether it might also be blocking legitimate, totally non-spamming comments. This is the issue of Akismet false positives.

I’ve just spent some twenty minutes or so going through the 371 comment items for this blog, as trapped by Akismet and being held for me to either clear or delets. Of the 371, one was a legitimate comment, which I cleared for publication.

Previously I had been looking at a sea of the usual spamming rubbish and hitting “delete all”. Now I will be taking the time to check, so that I don’t thoughtlessly delete legitimate comments.

Post from: EveryJoe

Akismet Catches Comment Spam Well But You Might Need to Check