WordPress 2.6 has
copied borrowed the best feature of Tumblr – their quick post bookmarklet that auto-detects photos and videos on a web page and enables you to quickly repost these items on your own WordPress blog.
The other useful features in WordPress 2.6 are Image captions, Google Gears support and Wikipedia style document revisions which come quite handy if you are working with multiple authors on the same post or want to revert to a previous version.
Matt has a detailed list of changes made in WordPress 2.6 though there aren’t any bug fixes or security related updates so am not sure if this qualifies as a must-have upgrade. You can download WordPress 2.6 here.