If you run a blog that accepts comments, you're probably familiar with comment spam. You could manually verify all comments, but that gets cumbersome after some time. A better solution is using an automatic scanner to verify comments. Like Mollom. Mollom checks your comments and provides CAPTCHAs if it's not 100% sure (about 2% of all comments).
The component is up to version 1.1.0 now and you can learn more about it (and find some downloads) on this page.