If you want to play an absolute gem of a game, go grab Cave Story. It's free, and it's "fun", in the most distilled form.
It's one of those games that makes you wish you could wipe your memory --- just to re-play it again for the first time.
http://www.actionbutton.net/?p=416
Why did Pixel make this game? Like, why the fuck? The amount of loving attention to detail that went into this game indicates that the man, beyond a shadow of a doubt, possesses the mental fortitude befitting a researcher in a facility in Antarctica, delving into the permafrost, seeking the Cure For All Cancers. Why did Pixel bother making a videogame, when God had given him this situational opportunity to Literally Save The World? At the very least, the man could have sought out the Ultimate Toothbrushing Solution — a three-second-a-day quick rinse that eliminates the need to brush your teeth, while also keeping them sparkling white. (It’d be like those antibacterial hand-wash-lotion things, only for your mouth.) Instead, he made a videogame.
I know around '07 there was a pretty large modding community making some real nice mods. I'm pretty sure it must be around still.
Also, Pixel (the creator of Cave Story) even made this program that lets you compose songs. It's a kind of tracker, but it's real good for some types of video game music, it's called PxTone.
For the first section or so, I didn't think it was all that fun, but after you get into it a bit, it gets pretty good.