SecureImage is nice, but the problem is that SecureImage doesn't work well with wp-cache, the cache plugin I use to insure a speedy experience for my visitors. So only a few minutes after installing SecureImage, I found another solution: reCAPTCHA.
The idea behind reCAPTCHA is very cool. It works by asking you to read two words, one which the computer generated and one that is part of a scanned page from a book, a word that the computer was not able to decipher. If you can understand the computer generated word, chances are that you'll also be able to decipher the scanned word. This helps digitize books! If you want to learn more about reCAPTCHA, I suggest you visit their site and read the "What is reCAPTCHA?" page.
I had to do a little tweaking to get it positioned properly with my custom WordPress theme, but that wasn't too bad. I'm really liking reCAPTCHA. Not only does it work with wp-cache, but it also fixes the annoying problem where your comment data would be lost if you enter the wrong information in the captcha. Of course, the captcha will be used by you, my visitors, more than anyone else, so please leave me your comments!