I like commenting on blogs, and I like it when people comment on my blog. The problem is that if comments are completely open, then spammers take over. WordPress has achieved an automated 99% solution to this problem with the brilliant Askimet software, but for other bloggers, each time we want to comment on what someone has written, we have to struggle though a CAPTCHA (Completely Automated Turing Test To Tell Computers and Humans Apart).
CAPTCHAs are evil because they discourage commenting (yes, even me), and that's a bad thing. But spammers are worse, so if we have to have CAPTCHAs, at least let them do something useful, like the new CAPTCHA from Carnegie Mellon University which allows bloggers to help digitize old books and manuscripts supplied by a non-profit organisation called the Internet Archive. Nice.
Update: In this interesting post, Tony discusses reCAPTCHAs and points at Google Image Labeler.