Cave Story, or Doukutsu Monogatari, is a superb 2D platformer created by Pixel and released at the end of last year. The downloadable game—for Mac or PC—is reminiscent of early NES platformers such as the Megaman series. The game is discussed in detail on the site, but I'll summarise it here. It will probably take you around 4-6 hours to complete, longer if you're inexperienced with these sort of games.
The game begins with a scientist trapped in a room sending a distress call to somebody named Sue. You then take control of your character, whom you know nothing of at the time, but gradually find out about.
After a brief introduction level, and the acquiring of a basic gun, you find yourself in Mimigas village, which will later turn into a hub of sorts. You discover that a Mimigas named Toroko is sheltering Sue, against the wishes of the other Mimigas. You also discover that 'The Doctor' has been coming and taking Mimigas from the village, leaving only six behind. After a brief encounter with the Doctor's minions, you begin the game proper.
There are lots of things to find and discover aside from the main story. There are a large number of Life Capsules scattered around, much like the in the Metroid games. These are used to increase your initially abysmal maximum health.
As well as the Life Capsules, there are also a number of optional weapons that can be gained, mainly by simple exploration. The weapon system itself is extremely interesting and noteworthy. When an enemy is defeated, they either drop hearts, used to increase health, or a number of orange triangles. When orange triangles are collected, your weapon's energy increases. When enough triangles are collected, your weapon gains a level, with a usually very notable change in firepower. The weapons can be built up to their maximum level of three very easily, but taking damage decreases your weapon's energy, so keeping all your weapons at their maximum levels can get somewhat tricky.
The controls are fairly simple and easy to use. Z is used to jump, X to fire, A and S to change weapons, Q to display the item screen and W to display the map if you have the Map Tool.
Unfortunately the game is download-only, but well worth it. Getting the game is fairly simple, but can require a number of downloads. Windows users must first download the game itself. Then, if you want you can download the game's music player. These two files are .lzh, but WinRAR can handle these files easily. The English translation patch is a thoroughly recommended .zip file. Mac users can follow the instructions found on this page.
Overall, Cave Story is excellently made and highly polished, with a superb storyline, brilliant gameplay and generally very enjoyable to play.