Item description:

XXSE is different from most save editors: all the editing and everything is done on a web page, right in your browser, and there's no program to run or download. You extract your saves with Modio/Horizon/whatever, edit them in XXSE, download and put them back. No saves are actually sent to or downloaded from the server, so you can (optionally) download the whole thing for offline use and run it on your own. Easy as any other program.

Supported games and notable things you can edit (things unique to this editor are marked in green):
Magic The Gathering: Duels of the Planeswalkers 2014
All deck and card unlocks, 10 promo FNM cards for each deck (!)
Premium foil decks
Sealed pool slot unlocks, and sealed pool editor for all slots. All 151 cards are available, filterable by color, type, and rarity. (!)
Campaign completion (opponents and encounters)
Revenge mode unlock and completion
Challenge mode completion
Misc stats
Halo 4
SP & XP
Commendations, with all values editable and the option to set each (or all) to levels 1-7
Campaign completion and mission unlocks
All armory items available as individually selectable options, including loadout weapons and slots, armor abilities, tactical packages, support upgrades, armor pieces, visors, stances, emblem foregrounds and emblem backgrounds. (!)
Title Update 7 armory unlocks
Magic The Gathering: Duels of the Planeswalkers 2013
All deck and card unlocks, including decks in the first two DLC packs and 10 promo FNM cards for each deck (!)
Premium foil decks
Campaign completion (opponents and encounters)
Revenge mode unlock and completion
Planechase mode unlock and completion
Challenge mode completion
Persona unlocks
Misc stats
FNM promo codes entered
Soul Calibur 5
All player stats, online & offline
Player title selection and individual title unlocks, including all 432 player titles
All character, stage, BGM, and mode unlockables
Story progress
Arcade stats
Quick battle progress, including stats for all 240 opponents
Custom character style, weapon, and name
Play time
Dark Souls
Most/all player stats and attributes, including hollow and curse states, sin, and New Game+ count
Covenants, including stats for each covenant
Equipped items and attunement
Individual warp bonfires enabled
Embers given to blacksmiths
Last bonfire rested at
Inventory and Bottomless Box - all items and all variations available (!); added items behave and sort correctly in-game; able to filter on item type, subtypes, and fully upgraded items
Bosses defeated
Gestures
Character appearance options
Magic The Gathering: Duels of the Planeswalkers 2012
Campaign, Archenemy, Revenge, Expansion, and challenge completion
Deck unlocks, card unlocks, and premium foil decks
DOOM (XBLA)
Health, Armor, Ammo, Ammo Capacity, Keys, Time, Kill Count, Item Count, Secret Count, Weapons Acquired, Current Weapon, and Power-Ups & Items
DOOM 2 (XBLA)
Health, Armor, Ammo, Ammo Capacity, Keys, Time, Kill Count, Item Count, Secret Count, Weapons Acquired, Current Weapon, and Power-Ups & Items including Computer Area Map, Light Amplification Visor, and Backpack
Grand Theft Auto 4
Gears of War
Campaign completion & chapter unlocks
COG tags collected
Chapters played as Dom
Misc stats, online & offline
Perfect Dark Zero
All weapon & cutscene unlocks
Mission completion, SP & co-op
Mission stats, SP & co-op
Deathmatch stats for all modes, including cumulative achievement progress
Weapon stats for every weapon, including cumulative achievement progress
Vehicle stats

Supported browsers:
Firefox
Chrome (doesn't work offline)
Opera (doesn't work offline, shows size of downloads incorrectly)
IE 10 (doesn't work offline, requires you to right-click and 'Save target as...' for save links)

Instructions:
Select your game from the list, and note the name of the file you will need in the file selection box.
On your Xbox, move or copy your save to a USB storage device. For Halo 4, Magic 2012/2013/2014 (Profile), and Gears of War, copy your profile.
Plug your device into your PC and open the save/profile using a program such as Modio (instructions are for version 5) or Horizon.
Extract your file:
Halo 4, Magic 2012/2013/2014 (Profile), and Gears of War: Extract the respective .gpd file from your profile. Halo 4 is 4D530919.gpd, Magic 2014 is 584112C5.gpd, Magic 2013 is 58411213.gpd, Magic 2012 is 58410B3F.gpd, and Gears of War is 4D5307D5.gpd.
With Horizon: Navigate to your profile in the device explorer, open it in the package manager, click the 'Contents' tab, and right-click the .gpd file and choose 'Extract...'.
With Modio: Click on your device, then your profile, then 'Profile Storage', then the 'E0000...' file, then 'Advanced', then 'Edit Package Contents'. Either filter for the game or find it in the list, then right-click it and choose 'Save as...'.
All other games: Extract the file (specified in XXSE's file selection box) from your save.
With Horizon: Navigate to your game save in the device explorer, open it in the package manager, click the 'Contents' tab, and right-click the file and choose 'Extract...'.
With Modio: Click on your device, then your profile, then your game, then your respective save file, then 'Advanced', then 'Edit Package Contents'. Right-click the file and choose 'Save as...'.
In XXSE, browse for the file you just extracted.
Modify as desired.
Click 'Save'
Click the link to download the modified file. You will need to right-click and 'Save target as...' with Internet Explorer.
Replace the respective file in your game save or profile with the one you downloaded, using the program you used to extract it.
With Horizon: Right-click the file in the package manager's 'Contents' tab and choose 'Replace', and select your downloaded file (its name doesn't matter). Do not use Horizon to modify Dark Souls or Soul Calibur 5 saves.
With Modio: First rename the downloaded file to match the file name as it exists in the save exactly, right-click anywhere in the list of files and choose 'Add File', select your downloaded file, and confirm the prompt to overwrite.
Rehash, resign, and save to your device.
With Horizon: Click the 'Save, Rehash, and Resign' tab. Do not use Horizon to modify Dark Souls or Soul Calibur 5 saves.
With Modio: Click on 'Done', then 'Save Changes', then 'General', and then the respective button to save to your profile and device.

Important: Horizon and some other Xbox 360 mod tools have issues replacing files in Dark Souls and Soul Calibur 5 saves, and will inevitably corrupt them. They may appear to modify them correctly, but issues will appear eventually - possibly rendering your data inaccessible. Modio 5 does not appear have any issues with these files.

Halo 4 Warnings: If you modify your XP beyond a certain threshold, you will be automatically banned. Do not make significant changes unless you intend to play offline only. Also note that for modifications to take effect, you must first load the game to the title screen and only then should you sign in. Signing in first then loading the game will result in it reloading your stats from the Halo 4 servers, wiping out any modifications you may have done. If you have gone online with modified stats, those changes will be permanent; stats cannot be lowered without all stats being reset the next time you go online. You should also note that for armory items to stay unlocked online, you must check the 'Override Requirements' box for each. Otherwise any items with requirements not met will be automatically locked or hidden while online.

Magic 2014 Note: To enable the sealed pool slots for editing in Magic 2014 (Deck Save), you must first open the sealed mode in the game at least once. If you unlock more than the first two sealed pool slots, you must load the game with the profile that has said slots unlocked at least once.