This is a new plugin of mine that will turn many existing WordPress blogs and their themes into Ajax-powered blogs. The best way to see what this plugin does is to look at the live demo.
The theme on that site wasn't tweaked at all to make the Ajax functionality work (except some irrelevant CSS changes). It works out of the box on most WordPress themes that follow the theme coding recommendations, see the plugin's readme.txt for details.
Downloads are available at WordPress.org.
WordPress plugin demos are disabled as I don't use WordPress on this site any more.