AttentionYou must play Doukutsu Monogatari. You do not have a choice. Your compliance is valued.
Seriously, this game has more hidden stuff than Puggsy, some viciously difficult boss fights, and a host of wonderful characters. It's a platform-based shoot-em-up, with influences from Metroid and Zelda. So far I've gotten the bad ending and the almost-good ending, but not the properly-good ending. Having found 3 different hidden weapons and almost finished it twice, I thought I was almost done. I found a guide to see if I'd missed much, and apparently I'm still missing the best weapon (although I'd figured out how to get it), some special power-ups and a raft of hidden stuff all required to access the secret "Hell" level, which I think is a prerequisite to getting the good ending. Yikes.
 - Relative to its size. This is huge when you consider it was done by a bunch of students, but it still can't compare to a full-sized game.