Cave Story (Various, 2004)

This one is pretty well known already, but sometimes I just like throwing something out there in case you managed not to hear about it.

Cave Story is a game made completely by one guy ("Pixel"). He did everything, coding, graphics, and even made his own music engine to write the soundtrack. (He also created another set of music tools called px Tone collage, which is more advanced, and most notably used by George and Jonathan)

I started playing this again because I was dicking around looking for stuff to put on my PSP and found a port of it. I never forgot how great it is, but I did forget how addictive it is. Once you get into it, you won't want to put it down until you've beaten it.

Cave Story is about a robot who wakes up in a cave with amnesia (or memory leak? is that a good joke?). You quickly find your self helping a village of Mimigas (kind of like little rabbit people), who are being kidnapped by The Doctor. He's looking for Sue, a human that's been turned into a Mimiga (which either was never explained or I just forgot the part where it was), and he needs her for his plans. There's a lot of story revealed through gameplay, so I don't want to give too much of it away.

It's available in a ton of formats, including Windows, OSX, PSP, GP2X, Linux, TI-83/84, and more, and there's an HD version available through WiiWare and DSiWare.