Some of you might remember the game "Knytt" by Nifflas (for those of you who don't, you can download it here on CWF). It was a mixture between platform and exploration game with wonderful graphics and atmosphere, but with a lack of interaction (with the inhabitants of Knytt's world) and action. All in all more a piece of art than a real game.
Now Nifflas has released a sequel called "Knytt Stories" (which takes place in the same world as "Knytt") and the game is beautiful as well, but much more challenging than original "Knytt". There are more monsters and a whole bunch of special abilities (upgrades) to be found now and there are three difficulty modes.
The basic game consists of a tutorial and one adventure ("The Machine"), but there are several new adventures (made by Nifflas himself or by fans) you can download from this place, on Nifflas homepage or in his forum.
And best is: "Knytt Stories" comes along with an easy to use editor. So if you still think the game is to easy then you can start to create your own adventures.
P.S.: Please look at the installation process. I haven't seen something that cute and funny for a very long time...
A more modern retake of Knnyt Stories, Knytt Stories+, is also available at http://egomassive.com/ks/