Opera 10 is the first release benefitting from their new Presto 2.2 engine. (Presto is Opera’s engine, as WebKit powers Safari and Gecko powers Firefox.)
There are a few new user interface features (spell checking, an option to automatically download and install future version updates, and changes to its emailing functionality), and changes to assist developers (improved and altered transparency capabilities, improvements to their Dragonfly developer toolkit), but the majority of the noteworthy items in this alpha release have to do with the rendering engine and overall speed.
You can learn more and download it here: http://www.opera.com/browser/next/