Freeware 2D Sidescrollers

I've been playing a good amount of 2D sidescrollers lately (all free), and I thought I'd compile a list of them, rating each one.

By Nifflas:
Within a Deep Forest (3/5 stars) *Good*
Nifflas' first game, Within a Deep Forest is a side-scroller about a bouncing ball with various powers. The gameplay is fairly well done, if not overly frustrating in some situations. The different powers of the ball create for some interesting gameplay situations, yet the challening difficulty and somewhat lackluster story doesn't make this game feel like a whole.

Knytt (4/5 stars) *Great!*
Nifflas returns to the platformer but this time he puts the emphasize on exploration, emotion, and sets it up on a colossal scale. The gameplay is quite simple, but proves rewarding as you explore this vast land with amazing attention to detail. The gameplay is nothing special, but the sheer emotional impact is enough of a reason to play this game.

Knytt Stories (3/5 stars) *Good*
Knytt Stories is close to an exact replica of Knytt, except with more gameplay features. These gameplay features include various powerups, more enemies, and various levels instead of one epic game. The focus of the game is still on exploration and emotion, yet it feels like the power-ups are just keys to unlock dead ends (large pits, cliffs, etc) to proceed through the game. Because of this, I did not enjoy exploring as much since most levels seemed to be very linear.

By Studio Pixel:
Cave Story (5/5) *SUPERB!*
Cave story is a relatively different game from Nifflas' as the emphasize is on story, characters, and combat. Cave Story is a very fun game that progresses well; all the characters develop and are well written, the story is revealed at the right pace, and you are constantly getting new weapons and gameplay features.
On the negative side, the first boss fights are a little too easy, while the last boss fights are too hard, usually requiring you to die several times to figure out the right strategy. Also, some of the optional features and better endings require you to do some inane things that I would never do without a walkthrough.

By Bernie:
A Game with Kitty (5/5) *SUPERB!*
A very fun and cute game where you play as a Kitty who must save the world by going to the moon and stopping an evil scientist. The gameplay is very fun, and the power-ups add tremendously to the experience. The only negative aspects are that the characters and story are absent in the middle sections and the difficulty gets insanely hard on the last level.

Darkside Adventures (AGwK 2) (4/5) *Good*
An indirect sequel to A Game with Kitty, Darkside adventures has a lot more charm to it, yet some of it is offset by some annoying problems. The player character is kind of a jerk, and the difficulty swaps between medium and impossible quite frequently. And this time around, there aren't as many power-ups making the gameplay more mundane, yet different.

DEFINITELY PLAY:
Knytt
Cave Story
A Game With Kitty