Another technique I find to be working really well is the "honeypot" technique. I create a CSS-hidden input field with a delicious, attractive name "url" and then validate it to be empty.
I use both the hidden honeypot and a random javascript injection that has to be matched server-side. Both have to pass.
The "problem" with this kind of tricks is that they works for small/medium website and only if they are not adopted as part as a big library that everyone uses.
They are not that hard to beat if you want to spam someone intentionally or if they are implemented by a well known plugin for (wordpress/joomla/etc..)
I use this technique a long time and it seems to beat all bots (a medium website, about 100k unique visitors per day). It's easy, unobtrusive and just works :)
