Hi,
I've been looking for a free extension with this caracteristics for a while now and this one looked good but it's quite old and i was wondering if it's still working on joomla 3.x, because it doesnt seem so for me but i don't trust myself much...

www.joomlaworks.net/forum/k2-en/30733-tabs-for-k2-free

I just need a free extension to filter my k2 items by an extra field selected by the user

Our own tabs and sliders has a new home:
www.joomlaworks.net/blog/item/264-tabs-sliders-has-a-new-home

In that post you will find links to download the new extension.