Good Form Security - no CAPTCHA
Try akismet. It's great at flagging spam. The API is easy to use and completely transparent to your users.
Here's what I've found to be very effective (and dead simple):
Put a hidden field on your form. Give it a name like "phone" or something similar/common and put in a default junk value.
Put another regular text input field on your form, but hide it with CSS. Make that one empty. Again, give it a "real" sounding name (first_name, phone_number, whatever).
When the form is posted, verify that the hidden field still has the default value and the field you hid with CSS is still empty.
You're basicly taking advantage of the fact that most spam bots will simply fill in every field in the form in order to avoid failing any required field validation checks. Some might be smart enough to ignore hidden fields, but I've never seen one that was smart enough to ignore fields hidden with CSS.
ETA: To address some comments - Is this a truly "secure" system? no, it certainly isn't. It would be trivially broken by anybody who wanted to specifically target your site. That said, it is still remarkably effective against the automated form spamming bots that most "low value" sites will see.
If you want to stop a determined attacker, you'll need something a bit more invasive. Another poster mentioned Akismet, which is a good option. Re-Captcha would be another. Stopping determined, targeted spammers is hard though. Even Yahoo and Google have a hard time with it.