UPDATES - PDF of the Machines but details are still scarce after a 7 minute "event" reveal of the machines. Gabe Newell is "deferring to the hardware vendors"
Possible Second Digital Storm Machine $2584 (GULP) // Digital Storm is "taking aim at the high end of the market, targeting consumers that demand the best possible gaming experience and who are looking for a PC capable of playing any title on their new 4K display.”- see Known Machines section
Webhallen Steam Machine (x3) - see Known Machines section
CYBERPOWERPC Steam Machine (x2) - see Known Machines section
Liveblogs will be available for the event. Here is a link to Ars Technica's Blog

Materiel.net confirms steam machine/reselling machines (poor translation) but claim's no further information for now as they are under NDA
Engadget have discovered 12 Steam machine partners "so far"
Alienware, Falcon Northwest, iBuyPower, CyberPowerPC, Origin PC, Gigabyte, Materiel.net, Webhallen, Alternate, Next, Zotac and Scan Computers are among the first companies signed on to support Valve's initiative, Engadget learned this afternoon.
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Throughout the week I will be doing my best to scour the web and put all the announcements and info here in the OP.
Valve will be kicking off their announcements with a press event on Monday 6th January at 5pm PT. Liveblogs avaialble. Information is scarce but the consensus is that the event will finally reveal who the hardware partners are and what products they will be bringing to market. Some media claim that software partners may also be revealed including games that will appear in 2014.
Valve have a slightly larger size booth than previous years so we can expect them and their partners to be showing off what they have both there and at each vendors' individual booths over the course of CES.
Last year's CES brought us surge of information regarding the Valve's plans for the future including the first confirmation of the Steam Machine project after Gabe was cornered in a toilet and asked for an interview. Hopefully there will be more of the same this year.
"Who is it for?" if old
Without further ado - "Thanks, and have fun"
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Known Hardaware So Far (ALL MACHINES STILL PROTOTYPES - cases subject to change
Webhallen Steam Machine
Inside a white BitFenix ​​Phenom houses the Intel Core i7-4771 with Geforce GTX 780 and 16GB of DDR3 memory. More details are motherboard Asus H81I-Plus, hybrid storage with Seagate Thin SSHD 1 TB, water cooler Antec Cooler H2O 620 V4 and power supply Corsair CX 600M 600 W. Webhallen however, planning to release both cheaper and more lavish options.
- The plan is to introduce three different models: a really cost-effective machine that can compete with traditional consoles and two machines for those who want a power enough to run everything on high settings. Of course these will cost a penny, but they may on the other hand offer performance on a completely different level than the recently launched game consoles, says Anton Nilsson.
CyberPowerPC Steam Machines (x2) - $499/$699
CYBERPOWERPC Steam Machine A - $499
Case: CYBERPOWERPC Steam Machine Gaming Chassis
Graphics: AMD Radeon R9 270 2GB GDDR5
Processor: AMD A6-6400K 3.90 GHz
Storage: 500GB SATA-III 7200 RPM HDD
RAM: 8GB DDR3 1600MHz Dual Channel Memory
Chipset: mITX motherboard w/ 802.11 WiFi + Bluetooth
Steam Controller
Steam OS
CYBERPOWERPC Steam Machine I - $699
Case: CYBERPOWERPC Steam Machine Gaming Chassis
Graphics: NVIDIA GeForce GTX 760 2GB GDDR5
Processor: Intel Core i3-4330 3.50 GHz
Storage: 500GB SATA-III 7200 RPM HDD
RAM: 8GB DDR3 1600MHz Dual Channel Memory
Chipset: mITX motherboard w/ 802.11 AC WiFi + Bluetooth
Steam Controller
Steam OS
iBuyPower Steam Machine - $499
Radeon R9 270, 500GB HDD, WiFi & Bluetooth, Steam Controller included
"Smaller than Xbox One" & "Existing Steam for Linux games should run quite well at 1080p 60 FPS"
Digital Storm Steam Machine (x2?) - $1469/$2584
Range of configurations will be available up to - GTX Titan, 700W PSU, dual boot with Windows, CPU Liquid Cooling & "Advanced Thermal Management"
4.4" x 14.1" x 16.4" (Slightly bigger than Xbox One)
""Rather than try to compete with console pricing, our system takes aim at the high end of the market and capitalizes on PC gaming's biggest advantage, raw performance""
Digital Storm is "taking aim at the high end of the market, targeting consumers that demand the best possible gaming experience and who are looking for a PC capable of playing any title on their new 4K display.”
Unconfirmed Steam Machines
PiixL JetPack - approx $1000
"Capable of housing overclocked Intel Core i7 processors, up to 1TB of SSD storage and the latest graphics cards from Nvidia, including the Titan and GTX780"
Mountable with TVs (32 to 70" VESA compliant), Walls & Tables.
**Claimed to launch on New years day, but has not. Also has never used the moniker "Steam Machine" - possible licencing issue/dodgy?
Not a Steam Machine
Xi3 Piston - Valve have distanced themselves from Xi3 since CES 2013 clamming "no involvement since"
Speculation
It seems a safe assumption to expect to hear about streaming boxes. There is a possibility that these may be helped along by AMD and Intel specific creations.
The Oculus Rift folks are out in force for CES this week after arguably being the star of the show last year. Valve has an ever growing collaboration with Oculus despite building their own VR prototype to be shown along with lectures from both companies at Valve's Steam Dev Days between Jan 15th and 16th. There may be some clarification about this at CES. Palmer Luckey remains coy about the situation:
Impressions and Vids of Valve's ongoing Prototype Steam Machine beta, can be found over here