Recommend something akin to Metroid or Castlevania
 

I'm looking for some games. I believe the proper designation is Semi-Open-Ended Action Adventure Platformer (RPG?). Yeah, I'm just going to refer to what is needed as Metroid'ish, or post-Symphony of the Night Castlevania...ish.

Any recommendations of similar games? Older or Newer, it doesn't matter. Just need something for a friend (a maybe a few for myself ^_^).

Accessible systems include: PC, PSP, GameCube, Gameboy DS, Gameboy Advanced, PlayStation 1 & 2, N64, and SNES.

There's a real fun game in this genre called "I Wanna Be The Guy". It's free for the PC, you should give it a try.

Cave Story is the best game in this genre, in my honest opinion.

Escape from Puppy Death Factory is a fun, browser-based one.

There's another good one that Donomni recommended me a while back that would be worth checking out.

Seconding Cave Story and I'll also add that the music is pretty kickass.

Also, I Wanna Be the Guy is cool and all, but it's only just this side of impossible. The only thing that the difficulty setting changes is the frequency of save points.

Cave Story. You can download the free version for PC or get the 2.5D remake for the 3DS that got released recently. The free version is pure SNES-style glory, haven't tried the remake yet. Either way, you have no excuse to not play it in some capacity.

Cave Story

Just pay above the minimum.

As one who's crazy about metroidvania - that's the genre you're looking for - games...

Look for:
-Bunny Must Die (PC, indie, Japanese but has an English translation ), is kind of hard, but not crazily so.
-Shaman King and The Nightmare Before Christmas ( both for GBA ) are exactly like the GBA Castlevania games.
-Monster Tale ( DS ) is pretty good, too.
-Iconoclasts ( PC, indie ), which is still being made, is shaping up to be really good.

Also, I'm sure someone out there must've made a Terraria dungeon that works exactly like a metroidvania game.

Demon's Crest for SNES is somewhat like that. It's from a time when Metroidvania wasn't really a word people actually used.

EDIT: Of course Cave Story. There's a free version. It's the full game. Not sure in which ways the paid version is different. http://www.cavestory.org/index.php

You might enjoy AnUntitledStory, free on PC. It's similar (get upgrades and unlock new areas) but VERY open-ended. It's got cute storybook visuals that hide a keen difficulty curve.

Paid version has remixed music on the Cave Story+ version. New music, but still great gameplay that I'm doing a playthrough on Youtube for.