Developer Metanet Software released a fun and unique 2D platformer some time ago where you play as a ninja (aka N) in a crazy, yet simple world containing 100 levels of platforming goodness.  The physics are very fun to mess with, especially in editor mode.  Likewise, you can easily build your own levels and play some great custom ones created by the large community.

This game has won plenty of awards, and it's easy to see why.  Visit Metatnet's download page and give it a try!