From the K2 website:
K2 is an advanced template for the blogging engine WordPress developed by Michael Heilemann, Chris J Davis, Zeo, Steve Lam and Ben Sherratt.
It won’t make you coffee, sing songs of sweet regret or sit at your bedside when you’re ill, but it might make life just a tad bit easier for you.
Think of it this way: Where WordPress is everything that goes on behind the scenes, K2 is everything that reaches the readers of your blog.
WordPress itself takes care of authenticating users, fetching and sending data to and from the database and provides you with the backend administration interface.
K2 on the other hand is the frontend of WordPress. It’s main concern is displaying the data fetches through WordPress in the right way at the right time. Furthermore, where more basic themes like Kubrick have little situational awareness, K2 cares about you and is always trying to make sure you are presented with exactly the tools and data you need.
K2 supports syles. Much like the main theme can be styled by using different CSS files.
K2 also has a stellar Sidebar Module(SBM) which is much like WordPress Widgets on steroids. Though, WordPress Widgets do have a more cleaner approach as they hook onto the WordPress Plugin API. On the other hand, SBM has a better UI to configure and more granular control for the users. Couple it up with solid API for the programmer. You can even disable SBM from the K2 admin panel to support WordPress Widgets.
So, here is my first SBM for K2.
Someone also had requested this as a sidebar module at the K2 forums.