I've never really been a fan of CAPTCHA as a way of checking that I am human. Unfortunately, I fully appreciate that they are an important way of cutting down on spam etc. There are a number of different CAPTCHA systems, some of which are better than others.
reCAPTCHA is my favourite system as I feel like I'm doing something useful as well as proving that I'm human. The aim of reCAPTCHA is to help in the digitization of old books. Put simply each CAPTCHA consists of two words; one is a normal CAPTCHA to which the answer is known and the other is a scanned word from a book which OCR couldn't properly recognise. If you can solve the first then they assume that you can solve the OCR problem as well. Once the same word has been presented to a few people they can determine the correct word with a high degree of accuracy.
They provide code and tools to help you add reCAPTCHA to your own site. They also provide a really simply e-mail hiding service. As an example here is a link to one of my e-mail addresses.