Cave Story is an awsome game... Plus it's free.
Note: Cave Story was made by 1 person. Take this into account while reading this/playing the game and you'll appreciate it even more.
Graphics: Cave Story's graphics look like a blocky GBA, and by today's standards are terrible. But they have a nice qualitey to them to. It is my personal oppinion that the graphics are perfect for what the game is.
Sound: The sound effects and music in Cave Story are about the equivelant of the graphics, but again have a very appealing side that's hard to describe. The music is catchy and I find myself humming it sometimes but it's also kinda repetetive.
Gameplay: The controls are convenient and easy to learn*. The game plays smoothly except the english mac version spontaneously crashes. (Note to self: Learn Japanese.) The design of the game is simple and there's a nice mix of killing stuff and talking to stuff.
Mechanics: Cave Story might be what you get if you crossed a 2D shooter with an RPG. You collect "Life Capsules" to increase your max HP (think metroid) and "Missile Expansions" for the # of missiles you can carry (think metroid). You also Level Up your weapons individually up to a maximum of LV 3 by picking up little orenge triangle thingies and lose EXP when you get hurt. Obviously, you try to keep all your weapons at 100% EXP and not get hurt because LV 3 weapons are much more powerful than LV 1 weapons (except the Nemesis, which is weaker the higher level it gets and fires rubber ducks at level 3, and Spur, which can NOT gather EXP and only levels up by cherging it AKA holding down the fire button)
There's tons of secred items, weapons, bosses and hidden endings for those of you who played and enjoyed the game too!
Overall, Cave Story is an awsome game and you should at leased try downloading it because it's free.
If you do wish to download it, I recomend the following fansite, where you can get it in japanese or english for Mac or PC (Make sure to seperately download and install the English Patch!): http://www.miraigamer.net/cavestory/
Another note: this is not my website and I am only advertising it because it's awsome.
*Arrow keys are move and aim, Z is jump, X is fire, A/S are qwitch weapons, Q is Inventory and W is Map.