This is an elegant filter pattern from skiplagged. At first glance, it looks like the sort of checkbox filter we see all the time. The bit I like is the “only” function that appears on hover. Clicking “only” unchecks all other options in the filter group. The feedback on how the filter is affecting the search results (-2 in this case) is nice too, but I’m not sure how obvious it is to the user.
The “only” filter isn’t unique, Kayak has it as well. It was featured in this 2009 article about Best Practices for Designing Faceted Search Filters–shame on me for not noticing. Nonetheless, it’s a nice pattern worthy of using in the future.