WordPress 2.6 contains a number of new features that make WordPress a more powerful CMS: you can now track changes to every post and page and easily post from wherever you are on the web, plus there are dozens of incremental improvements to the features introduced in version 2.5.
There are several new features in WordPress 2.6. The WordPress team have also created a video that demonstrates the new features bundled with WordPress 2.6.

Happy upgrading and gud lak!
[WordPress 2.6 - WordPress Blog]

Version 2.5.1 of WordPress is now available. It includes a number of bug fixes, performance enhancements, and one very important security fix. We recommend everyone update immediately, particularly if your blog has open registration. The vulnerability is not public but it will be shortly.
Here are some WordPress 2.5.1 highlights:

Performance improvements for the Dashboard, Write Post, and Edit Comments pages.
Better performance for those who have many categories
Media Uploader fixes
An upgrade to TinyMCE 3.0.7
Widget Administration fixes
Various usability improvements
Layout fixes for IE

[WordPress 2.5.1 - WordPress Blog]
Happy upgrading!