Spam Comments – random phrase plug-in

Is there a plug-in (like I’ve seen on all the other blogs) that makes a commenter fill out a verification phrase (like AV49UtX) to prove they are a person and not a machine? If so, how do I implement it on my blog?
Right now I require everyone to register, but I’m linking to a few more relevant places (larger and more read) and I think some readers want to remain anonymous, but still comment without registering.
Thanks,
Jeff