Just google it - it was a MS promotion in which they where sent out to various people. Each one had a URL and an unlock code. This viral web site loaded a flash game.
The flash games levels (actually actions) where all locked and each gear unlocked a level (not in sequence). Players can only proceed to the next "level" / "action" once this becomes unlocked by somebody else.
UPDATE on what is was: Microsoft's viral Gears of War 2 Last Day campaign - where codes found on real life metal COG tags are entered online to unlock Gears 2 related media - is heating up, taking the COG tag discovery to fans worldwide.
When first "announced", the Last Day campaign was presented in a way where we figured that the metal COG tags were only given to select media members and the codes (CSIDs) would only unlock various wallpapers, icons and posters on the Last Day website. Well, things are a bit different now (and still confusing), because now COG tags are being hidden all around the globe and it's your mission to find them, enter the codes online and unlock video content on the Last Day website. COG tag locations are randomly popping up on the website in the form of longitude / latitude coordinates, which you can view right here. There's still a bit of confusion and lack of organization, but you can follow all the latest Last Day COG tag news by reading the epicforums, keeping an eye on Gears 2 community manager SixOkay's Twitter and by browsing the official Last Day website. Find those tags!
The last tags where activate 2008.10.02 so I guess its kinda of a collectors items.