When working on a project that includes SharePoint in Internet facing scenario, your customer might request a CAPTCHA or some similar anti-spam algorithm to prevent automated postings to blogs and wikis.
I found a nice .NET component developed by LanapSoft. It includes some cool features like 50 different CAPTCHA algorithms and sound support. You can find the details on how to create a SharePoint web part with it here: SharePoint Captcha.
Update 25th May 2011:
There is a new version of the product (BotDetect CAPTCHA) mentioned in the original post this post. Here is the the newest step by step guide describing how to integrate BotDetect Captcha into SharePoint 2010.