Pennypincher's Corner I: Cave Story

Platform: PC
Genre: Action-Platformer
Developer: Studio Pixel
Publisher: Studio Pixel
Release: 20 December 2004

Cave Story is notable in that it was made by one person over the course of five years. It was a labor of love, and it shows. Cave Story is brilliant, and is in fact more well-made than many AAA titles. An updated version is available for download on WiiWare, but the original PC version of the game remains free.
As for the plot, the main character wakes up in a cave with amnesia, and saying anything more would be a spoiler. Half the fun of the game is simply figuring out what happened that lead up to the events occurring in-game. A brilliant cast of characters and lots of nice world-building make it a very worthwhile plot, as well.
Gameplay? Excellent. Comparisons have been drawn to Metroid, which is rather appropriate; the biggest difference from Metroid is in difficulty. Cave Story starts out rather easy, but quickly becomes downright brutal. And if you want the good ending, you have to work for it (you will not get the good ending your first time through, so don't worry about it and just enjoy the game).
While flawed (but isn't every game?), Cave Story is a game certainly worth experiencing. It can be downloaded here.

--Kotaro