"Extreme Keen" mini-levelpack by Perry Scanlon.  These levels are challenging, but not impossible:

Level 1: Get a bunch of extra lives and ammo.  And if you feel like it, you can grab the spaceship parts and beat the game.  Skip the parts if you want to play the real levels.

Level 2: A descending level.  Each landing brings a new surprise...

Level 3: Pyramid of flashing lights.  Guaranteed to annoy and frustrate.  :-)

Level 4: "Keen goes postal!"  You get to shoot lots of monsters.  Simple left-to-right level.

Level 5: A very short ice level.  The pogo stick helps.

Level 6: An ascending level.

Level 7: "Keen gets paranoid!"  Monsters and robots from every direction.

Level 8: Ice level with shooting robots.


For those who prefer mazes and puzzles:
http://www.geocities.com/cactus2clouds/keenpuzzle.html 
