Kinda new at the whole CAPTCHA thing, mostly becuase I have not had much of an issue with spam, but I have a client that is getting slammed with the crap. Can someone suggest a simple but fairly effective CAPTCHA.
Many thanks,
Houston
The best I’ve ever used is reCaptcha.
It’s accessible to the partially sighted, and works on a nice concept.
As far as I understand, all words are from scanned in books. The first word is known so is validated, and the second word stored in a database. If a lot of people put that text value as the second word, it is added to the dictionary.
By doing this, it is allowing people to scan in books and eventually get converted into digital books. Nifty, eh?
There are two very simple things you can do. One, google “spam honeypot”. Two, in your form just say “2 + 2 = ?” and with your php verfify that it equals 4 before sending.
I installed one once that used images and a dropdown box with about five options. You picked the option e.g. a photo of a dog and you picked dog from the drop down.
Simple and easy for anyone to use - as long as the understand english 
I can not remember where I got it from now.
Thank you guys for all the feedback. They are killing us with spam. WOW. never before have I experienced this.
I wrote one that presented four thumbnails and asked the user to, e.g., click the puppy. The four randomly selected images were stitched together into a single image in GD and an image map was generated. As I recall, there was some quirk in the way MSIE submitted the map coordinates that made it hard to get it working without JavaScript.
Try this World Wide Weird - no javascript 
Of course, you are making your website inaccessible to those who are visually impaired if you have an image-only CAPTCHA device.
I highly recommend this:
PHP Captcha by Securimage. It works great. It even has audio capability for the visually impaired.
You only have to add a few lines to your code to install it and verify it. It is free.
I use a prior version and I love it.