Hey. I've been doing this for the last weeks on another swedish forum, but what the heck - people are wondering about this all the time, so I'll do a quick translation and try to update it often here too.
Anyway, here's the updates for Market Place:

Demos:
Need for Speed: Most Wanted [2/12] - 1 GB
Kameo [2/12] - 500 MB
NBA Live 06 [2/12] - 500 MB
NFL Madden [5/12] - 600 MB
Fifa: Road to World Cup [9/12] - 500 MB
Condemned [29/12] - 500 MB
Quake 4 [30/12] - 600 MB - Multiplayer Live support
Fight Night Round 3 [5/1] - 500 MB
Project Gotham Racing 3 [10/1] - 1.2 GB
Full Auto [25/1] - 600 MB
King Kong [13/2] - ??? MB [Temporary down due to bugs in demo]
Call of Duty 2 [24/2] - 300 MB
The Outfit [6/3] - 450 MB
Ridge Racer 6 [9/3] - 600 MB [US Only]
Burnout Revenge [20/3] - 500 MB
Blazing Angels [23/3] - 600 MB
Battlefield 2 Modern Combat [29/3] - 400 MB - Multiplayer Live Support
Ghost Recon AW [30/3] - ??? MB - Multiplayer Live Support
Tomb Raider Legend [5/4] - 600 MB
Amped 3 [10/4] - 700 MB
Tiger Woods 06 [13/4] - 900 MB
Ghost Recon Advanced Warfighter Single player [4/20] - 700 MB
Kameo secod demo [4/24] - 1 GB
Top Spin 2 [3/4] - 500 MB
World Cup 2006 [3/4] - 650 MB
Test Drive Unlimited [5/8] - 500 MB
Lost Planet [5/9] - 500 MB
MotoGP 06 [5/11] - 900 MB
Hitman: Blood Money [5/25] - 800 MB
(All free to download)

Possible upcomming demos:
Timeshift: In this month's OXM US, will probably be up soon
Ninety Nine Nights: Delayed, date unknown
Perfect Dark Zero: Announced, release date unknown
Prey: Announced, release date 6/22

Xbox backwards comp:
Updated 29/3:
New games:
BLACK™
Star Wars Battlefront™ II
World Soccer Winning Eleven 9

Updated games:
Darkwatch™
Grand Theft Auto: San Andreas™
Half-Life® 2
Fable®
Fable®: The Lost Chapters™
Forza Motorsport™
Ninja Gaiden®
Ninja Gaiden® Black
SSX 3
Tom Clancy’s Splinter Cell® Chaos Theory™
kill.switch
World Series Baseball® 2K3.

Updated 9/12:
Tom Clancy's Ghost Recon® 2
Tom Clancy's Ghost Recon® 2 Summit Strike
Tom Clancy's Rainbow Six® 3
Tom Clancy's Rainbow Six® 3 Black Arrow™
Tom Clancy's Splinter Cell®
Tom Clancy's Splinter Cell® Chaos Theory™
Tom Clancy's Splinter Cell® Pandora Tomorrow™
The UK-EU list:
http://www.xbox.com/en-GB/games/backwardcompatibil...

Xbox Live Arcade:
New titles released after the launch:
Outpost Kaloki X [5/12] - 800p
Hardwood Backgammon [9/12] - 400p
Hardwood Hearts [9/12] - 400p
Hardwood Spades [9/12] - 400p
Wik: Fable of Souls [14/12] - 800p
Robotron: 2084 [16/12] - 400p
Marble Blast Ultra [25/1] - 800p
Crystal Quest [7/2] - 400p
Jewel Quest [8/3] - 800p
Feeding Frenzy [15/3] - 800p
Astropop [3/22] - 800p
Uno [5/9] - 400p
Game info:
http://www.xbox.com/en-US/livearcadexbox360/

Possible upcoming Arcade-games:
Street Fighter II Hyper Fighting - Announced, delayed
Texas Hold 'Em - Announced, release date unknown
Cloning Clyde - Announced, release date unknown
5 Midway games - Announced, release date unknown
E3 conference games - Announced, release date unknown

Patches/Downl. content:
Dashboard - Small update [1/30]
Cáll of Duty 2 - Single player patch [2/1], Multiplayer patch [3/22], Call of Duty 2 - Multiplayer maps [5/11]
Dead or Alive 4 - Gameplay tweaking [3/3]
King Kong - Small update [3/7]
Elder Scrolls IV: Oblivion - Horse armor [4/3] - 200p, Orrery map [4/17] - 150p, Wizard Tower [4/22] - 150p, Thieves Den [5/22] - 150p
Kameo - Co-op and two packs [4/6]
Project Gotham Racing 3 - Speedpack [4/14] - 400p, Cadillac cars [5/22] - Free
Major League Baseball 2K6 - General fix [4/27]
Perfect Dark Zero - New bots [5/4]
The Outfit - New maps [5/18] - 500p

Possible upcoming patches:
Perfect Dark Zero - Co-op patch & maps
Battlefield 2 Modern Combat - General patch

Others:
Rare christmas theme [13/12]
Ridge Racer 6 content [14/12]
Lost Planet trailer [14/12]
Mission Impossible 3 trailer [14/12]
NBA Live 06 theme & Gamer Cards [22/12]
Outpost Kaloki X-scenarios [22/12]
~30 different picture packs[30/12]
Ridge Racer 6 content [19/1]
Super Bowl dashboard themes [1/2]
Fight Night Round 3 themes & picture packs [13/2]
Crystal Quest content [13/2]
Hexic theme packs [13/2]
Bejeweled 2 theme packs [13/2]
ES: Oblivion themes & trailer [22/2]
Burnout Revenge picture pack, theme & trailer [27/2]
Dead Rising picture pack, theme & trailer [1/3]
GRAW developer diary trailers [1/3]
GRAW trailers [10/3]
Superman Returns trailer [13/3]
BF2MC picture packs & themes [13/3]
St. Patrick’s day picture pack & theme [14/3]
Top Spin 2 trailer [3/15]
Burnout Picture packs [3/15]
GRAW trailer [3/16]
The Outfit Theme and Picture pack [3/20]
GRAW theme [3/20]
GRAW trailer [3/22]
Oblivion trailer [3/23]
GRAW trailer [3/23]
The Outfit picture pack [3/27]
Dynasty Warriors 5 Picture packs [3/28]
BF2MC picture packs & theme [3/30]
Natasha Bedingfield video [3/30]
Tomb Raider Legend trailer [4/4]
Battlefield 2 MC trailer [4/4]
Ninety Nine Nights trailer [4/5] - Japan Only
Tiger Woods picture packs & theme [4/5]
Kameo Content packs [4/6]
Rumble Roses XX trailer [4/6]
GRAW picture pack [4/6]
Zegapain XOR-trailer [4/7] - Japan Only
Final Fantasy XI Trailer & Picture pack [4/10]
Jack Black Confessional series [4/12] - US Only
Nacho Libre Trailer, Theme and Picture pack [4/12]
MI:III Trailer, Theme and Picture pack [4/12]
GRAW Picture Pack 2 [4/13]
X-Men:The Last Stand Trailer [4/13]
Ice Age 2:The Meltdown Trailer [4/13]
Tomb Raider Themes and Picture Packs [4/13]
Burnout Revenge Car Unlocks [4/14]
Table Tennis trailer [4/17]
PGR3 free gamer picture [4/17]
DOA4 Which is Itagaki Trailer [4/19]
Blazing Angels Trailer [4/19]
MLB 2K6 Trailer [4/19]
GRAW Theme [4/19]
Nacho Libre Confessional Videos [4/20]
Xbox360 Community trailers [4/23]
Lost Planet pre-E3 trailer [4/25]
World Cup 2006 trailer [4/26]
Major League Baseball 2K6 theme & picture packs [4/27]
Dead Rising theme & picture pack [5/1]
BattleStation: Midway trailer [5/8]
Brother In Arms Hell's Highway trailer [5/8]
Bullet Witch trailer [5/8]
College Hoops trailer [5/8]
Dance Dance Revolution Universe trailer [5/8]
Dead Rising trailer [5/8]
John Woo Presents Stranglehold trailer [5/8]
NCAA Football 07 trailer [5/8]
Vampire's Rain trailer [5/8]
Sonic the Hedgehog trailer [5/8]
Test Drive Unlimited trailer [5/8]
The Lord of the Rings, The Battle for Middle-Earth II trailer [5/8]
Xmen Gamer pictures [5/8]
Adidas Gamer pictures [5/8]
Call Of Duty 3 trailer [5/9]
Culdcept SAGA trailer [5/9]
Hitman: Blood Money trailer [5/9]
Lost Planet Extreme Condition trailer [5/9]
Marvel: Ultimate Alliance trailer [5/9]
NBA 2K7 trailer [5/9]
Rockstar Games Presents Table Tennis trailer [5/9]
Tony Hawk's Project 8 trailer [5/9]
Alan Wake trailer [5/9]
Crackdown trailer [5/9]
Fable 2 trailer [5/9]
Forza 2 trailer [5/9]
Gears of War trailer [5/9]
Shadowrun trailer [5/9]
Too Human trailer [5/9]
Viva Pinata trailer [5/9]
Tom Clancy's Splinter Cell Double Agent trailer [5/9]
Enchanted Arms trailer [5/10]
Games For Windows Montage trailer [5/10]
MotoGP '06 trailer [5/10]
Prey trailer [5/10]
Call of Duty 3 trailer [5/10]
World Series of Poker: Tournament of Champions trailer [5/10]
Xbox 360 Games Montage trailer [5/10]
Xbox Live Arcade Montage trailer [5/10]
X-Men: The Offical Game trailer [5/10]
Dead or Alive® Xtreme Beach Volleyball 2 trailer [5/10]
Ninety-Nine Nights trailer [5/10]
WWE Smackdown vs. Raw 2007 trailer [5/10]
X-Men: The Last Stand Theme [5/10]
Gold Picture packs [5/10]
Silver Picture Pack [5/10]
Rainbow Six: Vegas Trailer [5/11]
Splinter Cell Behind the Scenes Video [5/11]
Adidas +10 Theme [5/11]
X-3 Gamer Picture Pack [5/11]
Xbox Live Silver Theme [5/11]
Xbox Live Gold Theme [5/11]
X-3 Theme 2 [5/11]
College Hoops 2K6 Theme [5/16]
NBA 2K6 themes [5/16]
Kameo trailer [5/18]
NHL 2K6 themes [5/22]
Theme/User pictures previews:
http://www.xbox.com/en-US/live/marketplace/gamerpi...

MS-points
This is the Market Place currency. The only way to get points is to buy them - Gamescore isn't the same thing. 1600p costs around $20 in stores, and if you order via dashboard the prices are:
500 points: $6.25
1000 points: $12.50
2000 points: $25.00
5000 points: $62.50

I'll update after your additions and my own searching.

Yeah thnx manager i'm really hoping that Saint's Row demo is coming, i can't wait to check that one out...

http://www.msxbox-world.com/xbox360/previews/previ...

"The online play is also something that will incorporate the gang element although the team were not wishing to talk about the live options at this stage other than a demo is planned to be available in the market place around Xmas this year. "

thnx for the new update manager....

I see you own a Xbox 360. As "you", you mean americans? Those demos are available in europe too, and for the record, I live in europe too.