Nifflas has reached out to announce that Knytt Underground will be arriving on Windows and Mac shortly -- possibly as soon as December 21! The game has already been available on PlayStation Network, but many of us know and love the series on the aforementioned platforms. That said, from what I've played and seen, you can't really go wrong either way.
Interestingly, it's not available on Steam yet, though the game does have a Steam Greenlight page which could use a spare upvote. As such, I would expect a direct release is on the cards for the time being. Give Underground a look if you're in the mood for a relaxing, exploration-based platformer with the occasional puzzle thrown in.