If you thought built-in widgets were all you got with WordPress 2.2, Austin Matzko of Il Filosofo has some details that might interest you.
You can now set your site’s “home” and “siteurl” values in the wp-config.php file
WordPress now uses PHPMailer for email
This is a great addition to the software, and if you want details on how all these things work, and what they mean, be sure to check out Il Filosofo.
The most useful currently for me would have to be controlling the home and site URL in the wp-config.php file. A great way to help move sites from one place to another.