With over 2,000 code commits and 820 bug and feature fixes, WordPress 3.1 has been released. You can download the new version in its entirety, or simply log into your WordPress.org installation and update via the Dashboard.
The most notable new new change is a massively overhauled writing interface, which should make the entire publishing process both more streamlined and less intimidating for new users. Linking back to old posts and pages is now a lot easier, too (just click a button and start searching), and lots of rarely-used blocks have now been hidden by default (they can be re-enabled in Screen Options).
There's a new Admin Bar, which appears at the top of every page and makes moving around your blog's back end a whole lot easier. For power users and theme makers, there are now Post Formats, which significantly increase the flexibility of WordPress layouts.
For a complete list of the changes and new features, check the WordPress 3.1 Codex page.