This is the first release of the HTML Purifier module compatible with Drupal 6.x! HTML Purifier 3.1.0 is required with this release.
The most prominent new feature is the revamped system for configuring HTML Purifier. Whereas the 5.x version has a few ad hoc form elements for some of the most common configurations, 6.x lets you access all of HTML Purifier's directives with a variety of methods. You can still use the simple form (which has been extended and expanded), the advanced form (which contains all web-editable configuration options in HTML Purifier) and the advanced PHP file system (which lets you perform extremely complex customizations to HTML Purifier).
The version also boasts a cache for HTML Purifier which prevents cache slams from filter's pitifully small three day expiration period, and an improved install and uninstall system.