I recently updated My Two Cents. My Two Cents (or MTC) is a simple PHP class to add comments to any PHP driven site. It is was written with simplicity in mind but still has some strong features and is easily tweakable. It consists of a single PHP file and can be added with just a few lines of code.
The post also describes some of the functionality it comes with, including a CAPTCHA mechanism and a blacklist feature to help reduce spam. There's also a bit of multi-language support included as well.