WPEngineer Michael Preuss walks you through the new simpler Widget API in WordPress 2.8 to create a simple widget. Very simple and easy to understand tutorial.
This is another great resource to learn how to build a widget using the new Widget API by Justin Tadlock.
If you find categories and tags too restricting for the stuff you write, Justin Tadlock explains how to define your own taxonomies in WordPress 2.8 to label your posts in the way you want.
WordPress 2.8 supports location-specific classes for body tag with which you can control the looks of almost everything with just CSS.
The WordPress codex explains the new author meta information methods, its parameters and examples.
WordPress 2.8 includes interesting register_setting() function that lets plugin authors leave the dirty stuff to WordPress and concentrate on the functionality of plugin.
If you are using WordPress on your local network which is behind a proxy, then now you can easily fetch content from other websites or RSS feeds as WordPress 2.8 requires just a little edition to wp-config.php to work behind a proxy.
If you know of some other nice tutorials on WordPress 2.8, feel free to share them with us.