Anti spam
I’ve been using Knowspam.net for about 4 months now, and I like it muchly. I moved to Mozilla Thunderbird for my email today, and was having a problem sending email through Knowspam. A question, a speedy and correct reply (even though the problem was unusual) that fixed it. Excellent. Well worth the small yearly charge. And: I get almost no spam these days.
The system challenges people to prove they’re humans before it lets email pass through to you. Most people never have to do that though (people already in your contacts, or people YOU send email to don’t have to), and the others only have to do it once. Sure, one or two emails will get lost now and then, but very few. And it has stopped the thousands of spam messages I was receiving a day. Highly recommended.