You can Download WordPress 3.0(Thelonious) now, there are a lot of update on the latest version of WordPress. This some of them new default theme called Twenty Ten. For Theme developers have new APIs that allow them to easily implement custom backgrounds, headers, shortlinks, menus (no more file editing), post types, and taxonomies, this is the happy news for some Theme Developer such WPGPL.com or Jauhari one of Theme Developer from Indonesia.
For the developers and network admins will appreciate the long-awaited merge of MU and WordPress, creating the new multi-site functionality which makes it possible to run one blog or ten million from the same installation. As a user, you will love the new lighter interface, the contextual help on every screen.
The other hand, on WordPress 3.0 has been fixed 1,217 bug and feature enhancements, bulk updates so you can upgrade 15 plugins at once with a single click. Are you ready to play around with WordPress 3.0? Download and enjoy it
For the last one, enjoy the Video Tour of WordPress 3.0
For Theme and Plugin Developer you can check the Codex for WordPress 3.0 here