Of the new features, two are notable for most end users.
Joomla upgrade adds ability to change username
First is the ability to control whether or not a user can change their username when editing their profile in the frontend.
As you can see below, there is now an option in the User Manager to explicitly allow or disable feature depending on the needs of the site.
To access these options, select User Manager in the Joomla administrative backend and select Options in the toolbar at the top-right.
Image upload improvements
The second interesting feature in these Joomla upgrades is the ability to simultaneously select and upload multiple images in the media manager. Previously, this was not possible unless you were using the flash uploader that was not universally supported by common server configurations.
In Joomla 2.5.5, this feature is now standard, meaning that almost every site can now easily upload numerous images at once without having to process each one individually.
If you are performing a Joomla upgrade from an existing Joomla 2.5 site, simply use the built-in updater to apply Joomla 2.5.6. For more information on Joomla 2.5.5 and 2.5.6, please read the official release announcements as well as the blog post that describes some of the other new features in more detail.