edit: Opps! Theres 240 not 150 Developers for the Xbox. Direct link:
http://www.xboxaddict.com/xboxinfo/developers/ 
2015
Acclaim
Activision
Adrenium Games
Aki
Alfa System
Altar Interactive
Anchor
Angel Studios
Arc System Works
Argonaut Games
Arika
Artdink
Artoon
Arxel Tribe
Ask
Atlus
Attention to Detail
Awesome Developments
bam!
Bandai
Barking Dog Studios
Bethesda Softworks
Big Blue Box
BioWare
Bizarre Creations
Black Box Games
Blitz Games
Blue Byte Software
Blue Shift
Brat Designs
Bungie
Bunkasha Publishing
Capcom
Charybdis
Cinemaware
Climax
Codemasters
Confounding Factor
Conspiracy Entertainment
Core Design
Crave Entertainment
Criterion Software Ltd.
Cryo
Crystal Dynamics
Crytek Studios
DarkWorks
Datam Polystar
Daydream Software
Deep Red Games
Digital Anvil
Digital Extremes
Digital Illusions
Digital Leisure
Digital Rim
DigitalWare
Dma Design
Dragonstone Software
Dream Factory
DreamCatcher
DreamForge Entertainment
Dwango
Dynamix
Edge of Reality
Eidos Interactive
Electronic Arts
Empire Interactive
Eon Digital Entertainment
Epic Games
Epix Interactive Studios
Escape Factory
Fantasy Studio.
Fox Interactive
From Software
Funcom
Gameplay
Gathering of Developers
Genki
Global A Entertainment
h.a.n.d.
H.I.C.
Hasbro Interactive
Havas Interactive
Headlock
Heavy Iron Studios
High Voltage Software
Housemarque
Hudson Soft
Humongous Entertainment
Hypnotix
I-Imagine Interactive
id Software
Image
Imagineer
Incredible Technologies
Infogrames
Innonics
InterAct
Interplay Entertainment
iRock Interactive
Jaleco
Kaboom Studios
Kalisto Entertainment
Kemco / Kotobuki System
Kodiak Interactive Software Studios
Koei
Konami
Kool Kizz
Kuju Entertainment Ltd.
Larian Studios
Legend Entertainment
LightWeight
Lionhead Studios
Lost Boys Interactive
LucasArts
MadCat Interactive
Madcatz
Majesco
Mass Media
Massive Entertainment
Mattel Interactive
Max-International
Mediavision Corporation
Metro3D
MGM Interactive
Microids
Microsoft
Midas Interactive Entertainment
Midway Home Entertainment
Monolith Productions
Monster Games
Mucky Foot
Mythos Games
Nagi
Namco
Nest
Neversoft Entertainment
Nihilistic Software
Nihon Create
NovaLogic
NuClearVision Entertainment
Oddworld Inhabitants
Over Works
Pacific Coast Power & Light
Panther Software
Panther Software
Papyrus
Paradigm Studios
Paradox Entertainment
Pharaoh Productions
Pipe Dream Interactive
Pitbull Syndicate
Pivotel Games
Prey Digital Studios
Pseudo Interactive
Qube
R A C
Radica Games
Radical Entertainment
Rage
Rainbow Studios
Raven Software
Rebel Act Studios
Rebellion
Red Storm Entertainment
Reflections Interactive
Remedy Entertainment
Revolution Software
Ripcord Games
Riverhillsoft
Rockstar Games
Ronin Entertainment
Saffire
Sandbox Studios
Savage Entertainment
SCi Entertainment Group PLC
Sega
Shiny Entertainment
Shoeisha
Sidhe Interactive
Sierra
Silicon Dreams Studio
Simon & Schuster Interactive
Sky
Smartdog
Smilebit
SouthPeak Interactive
Spike
Starbreeze Studios
Stormfront Studios
T&E Soft
Taito
Takara
Take-Two Interactive Software
Takuyo Kougyo
TDK Mediactive
Team Andromeda
Team Ninja
Tecmo
Telenet Japan
Terminal Reality
Testing
The Collective
The LEGO Compay A/S
The Pitbull Syndicate
The Whole Experience
THQ
Thrustmaster by Guillemot
Tiburon
Titus
Titus
To Be Announced
TopWare Interactive AG
Totally Games
Treasure
Tremor Entertainment
Treyarch
Twilyt Productions
Ubi Soft Entertainment
Valkyrie Studios
Valve, LLC
Victor Interactive Software
Video System
Virgin Interactive
VIS entertainment PLC
Vision Games
Volition, Inc.
VR-1 Entertainment
Warthog PLC
Westwood Studios
Wow Entertainment
WXP
XeNN
Xenopi
Yager Development
Yuki Enterprise
Z-Axis
Zetha gameZ
Zombie VR Studios
Zono