For the impatient, grab the update here: http://getk2.org/downloads/?f=K2_v2.6.6.zip
If you are on Joomla! 1.5, grab the zip file from the link above and install it on top of your existing K2 version. If you are on Joomla! 2.5 or 3.0, go to the Joomla! update manager, purge the update cache, re-check and you'll see the new release available to instantly update. If you are on Joomla! 1.6 or 1.7 then someone probably cut off your internet connection a year ago: upgrade to 2.5 now!
So what's changed or been updated in this new release in more detail?
First off, we fixed 2 bugs related to the new advanced SEF options. For existing sites that switched over to the new "Advanced SEF" options introduced in K2 2.6.4, the URL redirection from the old URL scheme for items to the new one was not working. Additionally, if the item id was removed via these options, then attachment URLs would simply fail to serve the downloads, as well as item edit forms in the frontend. All that is fixed now.
We've also added some internal performance improvements and have completely removed the embedded script tag that defined the site's root path (which was used by the rating). K2 will now load 1 CSS and 1 JS file only and it does not embed any JS in the <head> of the template. It will also call the Joomla! modal only when needed (e.g. on frontend editing or in the K2 item page for the xlarge image preview).
All K2 feed pages generated automatically will now properly validate their output in either Atom or RSS format. A simple ampersand (&) in the K2 category's name would previously cause that category's feed to fail validation.
In the backend, we've added a handy toolbar button to quickly delete any orphan tags, aka tags that are no longer associated to any K2 item. The button is called "Delete orphan tags" and it will make sure your tags database is kept tidy. Furthermore, we've added a new control to filter the tags list by the number of items they are linked to. That way you can easily spot popular vs. unpopular tags in the backend.
We have also added a new "language" colum in K2 items and categories so it's easier to spot items/categories that belong to different languages when you're not filtering a specific language.
The quick icons will now properly display in Joomla! 3.0's control panel (dashboard).
Finally, we have added support for the upcoming Simple Image Gallery Pro version 3, which allows for ultimate image gallery control from within K2's "Image Gallery" tab, including caption/label editing! See what we wrote on the JoomlaWorks blog about Simple Image Gallery Pro version 3. You'll be blown away! :) This new release is coming out this Wednesday, April 10th 2013.
There are no template changes between 2.6.6 and 2.6.5.
For a full list of changes, grab a coffee and check out the list from our SVN server: http://code.google.com/p/getk2/source/list
If you notice a bug, please take a moment to report it here: http://code.google.com/p/getk2/issues/list - we check this list more than email :)