I've "made" a few video games, by taking existing NES games and radically altering them.* 

This is called ROM Hacking. Only a few I've done are really worth mentioning:

 

Major Hacks

Dragoon X Omega v2.0f

A hack of a hack of Dragon Warrior. This game is much darker than the original, and is sci-fi instead of fantasy. Post-apocalyptic fun for all.

 

Dragoon X Omega II - Final 2

A hack of Final Fantasy, done with another ROM hacker named Thaddeus: He did 99% of the artwork, and level design, which is amazing: Far beyond anything I can produce. I did the soundtrack, storyline, battle engine, some programming, gameplay mechanics/etc. Several other talented people gave us a hand as well who are mentioned in the readme.

  

Minor Hacks

Castlevania II: Belmont's Revenge Speed Hack v1.1


 

In my opinion, the biggest weakness of the Gameboy Castlevania games was the slow speed of the protagonists. While this made sense due to the blurring issues the Gameboy LCDs have, on an emulator it's just annoying. This boosts the speed of Christoper, and contains patches for every release of the game.

Castlevania - Legends: Speed Hack v1.1

This patch serves the same purpose at the Belmont's Revenge hack. Large portions of code are shared between Legends and Belmont's Revenge, so identical routines were hacked. This means Sonia and Christopher move at identical speeds. Includes patches for the USA and Japanese versions of the game.

Recca - Pure

Recca is, in my opinion, the best shooter to grace the NES. What most people don't know is that there are two hidden modes to the game: A hard mode with 7 levels (As opposed to the 4 in the standard game) and "Zanki Attack", which is basically either normal or hard mode with enemies that explode into bullets!

 Recca - Pure allows both to be accessed from the title screen, adds a few new graphics and also translates the Sound Test menu

The original patch replaces the title screen with a Sol-Deace inspired one, while the alternate patch changes the title screen to one more faithful to the original Recca title.

 

 Translations

Valis - The Fantasm Soldier v1.1

 

This is a 100% complete translation of the NES Valis. satsu joined me for the translation part of the hack, while I did the technical work.
Version 1.1 fixes a major bug that could crash the game in the 2007 release.

 Valis++ v1.1

I always thought I could never make it far in Valis due to something I couldn't understand because of the whole language issue. It turns out it just has some horrible design flaws.

 Valis++ includes the translation above, as well as some assembly hacks that make the game *far* nicer to play. If standard Valis is making you rip your hair out, this may be right up your alley.
Version 1.1 fixes a major bug that could crash the game in the 2007 release.

 Gravity Armor Metal Storm v1.01

The japanese version of Metal Storm had a cinematic intro and a different ending. This makes them english!

*The games themselves are patches: You must provide the original games and apply them with an IPS patching program. The resulting ROM image then must be run on an emulator. There are a plethora of patchers and emulators for Win32, Linux, and MacOS. Please do not ask me for the original game ROMs, as I cannot, and will not, provide them to you.