I've implemented the HIP-CAPTCHA control that Miguel Jimenez put together. I deleted about 200 spams this week and had finally had enough. Seems to work just fine. My only feedback for Miguel might be that the text is hard to read - I don't know if it's a combination of the colors or if it's the size.
Help me out and test it by commenting on this post. I'd be especially eager to hear from firefox/netscape/mozilla users as well as those on non-Windows platforms. There shouldn't be any compatibility problems as the code runs on the server where the jpg is then rendered and pushed down as HTML. Always pays to be cautious, though!