Year Zero (also known as Halo 24 or Year Zero: Zero-Gen-Ten in Japan) is the sixth studio album by Nine Inch Nails, released on April 13, 2007 in the UK and Europe, and April 17, 2007 worldwide, by Interscope Records. It was produced by Nine Inch Nails frontman Trent Reznor in collaboration with Alan Moulder and engineer Atticus Ross, both of whom worked in a similar capacity on Nine Inch Nails' previous major studio release, With Teeth. Mastering was done by Brian "Big Bass" Gardner.
In a 2005 interview with Kerrang!, Reznor expressed his intentions to write material for a new release during the With Teeth tour, and it was confirmed that he was working on it by September 2006. Year Zero began its mixing stage in January of 2007, and Reznor stated on his blog on The Spiral that the album was finished as of February 5, 2007. The album marks a new style of lyric writing for Reznor:
"...for the first time in my life, I wanted to break away from the idea of opening up my journal and transferring that into song lyrics. And I really thought about what was at the forefront of my concern, and, at the state I'm at right now, at the age I'm at, just the state of being an American citizen, a lot of concern I have about the direction the country's headed in, and uh, kind of the erosion of freedoms that seems like we're experiencing, and the way we treat the world and our own citizens felt like something I needed to comment on." 
A subsection of nin.com, yearzero.nin.com, was created and contains various things Year Zero-related. An alternate reality game was constructed by Trent Reznor and Rob Sheridan, then run in partnership and developed by 42 Entertainment to provide listeners with a greater understanding of the album's concept. Reznor paid for the ARG himself, trying to keep it a secret from his record company as much as possible, stating: "What you are now starting to experience IS 'year zero'. It's not some kind of gimmick to get you to buy a record - it IS the art form... and we're just getting started. Hope you enjoy the ride." The game ran from roughly February 12, 2007 to May 2007, beginning when a string of mysterious websites was discovered around the internet related to the album; the game has since been nominated for two Webby Awards. Learn more at the Year Zero Research page and view the timeline of discovery for an explanation of how things unfolded, also, 42 Entertainment has a complete overview of the project here.
Trent Reznor on Year Zero
"This record began as an experiment with noise on a laptop in a bus on tour somewhere. That sound led to a daydream about the end of the world. That daydream stuck with me and over time revealed itself to be much more. I believe sometimes you have a choice in what inspiration you choose to follow and other times you really don't. This record is the latter. Once I tuned into it, everything fell into place... as if it were meant to be. With a framework established, the songs were very easy to write. Things started happening in my "real" life that blurred the lines of what was fiction and what wasn't. The record turned out to be more than a just a record in scale, as you will see over time.
A1 "HYPERPOWER!" - 1:42
B1 "Capital G" - 3:50
C1 "The Greater Good" - 4:52
For more information, see Performance 2007
The album was supported by a tour that visited Europe, Asia and Australia, with an emphasis on locations never or seldom played by NIN. The production for this tour was scaled back and setlists became very fluid as a result. More of the NIN catalog was able to be covered on this tour, as several songs were played live for the first time and several songs were brought back to the setlists after a long absence. As the tour progressed, more Year Zero songs were added to the setlists.
Year Zero Hints
For full details on all hints found through the retail release of the CD see this ETS thread.
Regarding the leak
On April 1, 2007, a bootleg recording of the Chicago, Illinois listening party of March 18, 2007 surfaced on several peer-to-peer networks. The quality of the recording is very low and it is apparent that whoever made the recording did not have very good sound capturing equipment, presumably a cellphone.
On April 4, 2007, a high-quality leak of the promotional disc supplied to listening parties was also leaked onto peer-to-peer networks. Perhaps to counteract and discourage any piracy of the leak, the entire album was made available for free streaming on the album's official website later that afternoon. Shortly after the stream was made available, high-quality leaks found their way to various sites and P2P networks.
Perhaps in another response to the leak, Reznor commissioned Q101 in Chicago to play the entire album on their radio station.
It is widely rumored, however, that Reznor himself was responsible for leaking the album as a result of his unappreciation for RIAA's Cease & Desist warrants issued to various websites that hosted free downloads of the 4 USB "leaks". Reznor's own involvement with the leak, however, will always be speculation at best due to his own legal liability.
Reznor has stated that multitrack files for all the songs on the album will be released, making them available for fans to remix for free. In March 2007, the multitrack audio files of the first single, "Survivalism," were released in Garageband format on the official Year Zero website for fan remixing. The multitrack files for "Capital G," "My Violent Heart" and "Me, I'm Not" were then released on April 26; those for "The Beginning of the End," "Vessel" and "God Given" were released on June 12. The formats were for Garageband and Logic Pro, and there were also WAV files for other applications, with the latter distributed through BitTorrent. The entire album was released in multitrack form on the Y34RZ3R0R3M1X3D album.
Reznor has stated that Year Zero is “part of a bigger picture of a number of things I’m working on. Essentially, I wrote the soundtrack to a movie that doesn’t exist."
In March of 2007, Kerrang! Radio reported that "Reznor admitted he’s already in talks about a movie version of his upcoming album, a concept piece, with part two scheduled next year." Reznor has said that he is currently more interested in a television project rather than a film and that he has a producer and has met with writers."
On August 14, 2007, he announced that he and his creative team would soon be taking the concept to television networks to try and make some sort of deal: "We're about to pitch it to the network, so we're a couple of weeks away from meeting all of the main people, and we'll see what happens." 
On June 8, 2008, The New York Times reported that "Mr. Reznor has been working with Lawrence Bender, Quentin Tarantino’s longtime producing partner, to create a cable TV series to tell the entire story. " 
On April 2, 2009, Reznor announced via a short Twitter post that "YZ is alive".
On September 28, 2010, The LA Times reported that the Year Zero television series detailed in the above NY Times article was going into pre-production, with Daniel Knauf as a writer and Lawrence Bender's team as staff. The miniseries is being handled by BBC America and will be broadcast on HBO. 
On March 10, 2011, Trent confirms on the nin.com board that work on the TV series is still in progress. 
On June 24, 2011, "Pitchfork Magazine" had a section of news stating that Jim Uhls, the man who had written the screenplay for Fight Club and the screenplay for the 2008 film Jumper, was chosen to write the script for the series.
On December 23, 2011 in an interview by "The Guardian" Trent is quoted as follows about the 10-part HBO mini-series, which is based on Year Zero: "I chose not to write it myself because I don't know how to do that. But now I'm thinking I should have. Somebody's got to do it, right? How hard can it be?" 
On January 11, 2012 a user on Echoingthesound said that Jim Uhls confirmed him via Facebook that he is currently working on the Year Zero TV series ("I've been away from FB for awhile. Now that I see it, I can answer. I am currently working on Year Zero. [...]" )