I can think of at least a dozen games in this category, but hopefully this genre is niche enough that we can manage to make a catch-all for it.
So the idea is to create and maintain a list of every game as described in the subject line of this thread and to talk about how great these games are and eventually I'll go through and put in links.
Master Alphabetical List:
Aliens: Infestation (DS)
Barnyard Blast: Swine of the Night (its straight linear action, but it's such a great parody of Castlevania games, and I was a big fan of the newsfeed/blog that coined the sub-title that the creators then actually used, what can I do?)
Castlevania: Aria of Sorrow
Castlevania: Circle of the Moon
Castlevania: Dawn of Sorrow
Castlevania: Harmony of Despair (timed multiplayer)
Castlevania: Harmony of Dissonance / White Night Concerto (GBA)
Castlevania: Mirror Of Fate
Castlevania: Order of Ecclesia
Castlevania: Portrait of Ruin
Castlevania: Symphony of the Night
SNES Demon's Crest
Dust: An Elysian Tail
Hasslevania: The Quest for Shuteye
Legend of Zelda
Legend of Zelda: A Link To the Past
Legend of Zelda: Oracle of Ages
Legend of Zelda: Oracle of Seasons
Legend of Zelda: Link's Awakening
Legend of Zelda: Minnish Cap Adventure
Legend of Zelda: Phantom Hourglass
Legend of Zelda: Spirit Tracks
Megaman Zero 1-4
Metroid II: Return of Samus
Metroid: Zero Mission
Muramasa: The Demon Blade
Ori and the Blind Forest
U.N. Squadron - you pick up new weapons, planes, and money and choose what missions to take on. More importantly I really love it.
Should these be on the Master List?
I've decided from now on we need two votes; more or less. Or I could be less lazy and do research.
An Untitled Story
A.R.E.S. Extinction Agenda (on Steam)
Aztaka (on Steam)
Biozone (on Steam)
LostWinds: Winter of the Melodias
Megaman games with ZX in the title
PixelJunk Shooter 1 or 2
What will not be put on the list and possibly a list of reasons.
Megaman games in general
1) too many games to mention
2) most games with the word Megaman in them do not fit the category and
3) if you've never heard of Megaman how did you get on this site?
Pure Platformers like Mario or N or Super Meat Boy
Pure Action games like Alien Hominid, Dark Void Zero, Contra, and Metal Slug
Anything with Turn-Based Combat
Batman: Arkham Asylum because it is in 3D
Criteria/Things to Keep in Mind:
Non-linearity is not strictly a requirement given sufficient space to explore
Must have combat as an option, we explore because we're looking for loot and things to shoot
Leveling up not strictly required either
(mostly) 2D gameplay
Freeform backtracking - i.e. you can backtrack if you want
Collecting money that you can spend at a shop is in Castlevania but not Metroid
Unlocking new equipment or abilities over time happens in a lot of games, but Mv does it in a way that allows you to change how you play the game, this is what separates this genre from (mostly) pure action games and pure platformers
Whew! That was almost like work. Post away, Metroidvanians, with games not on this list, why they should be included and your praise for the same.
Should I put "adventure" in the title. For instance, if not for the fact that Metroid is in the title of metroidvania, Metroid is really an adventure game. How does one tell the difference? To me, in this case, Metroid, Legend of Zelda, and Hasslevania provide discrete power-ups while Castlevania provides a variety of weapons with differing characteristics and a continuous character progression system.