The offer, which runs throughout December, enables free Live multiplayer for 8 XBLA games: Super Street Fighter II Turbo HD Remix, A Kingdom for Keflings, Aegis Wing, Hardwood Hearts, Hardwood Spades, Small Arms, UNO and Bomberman Live. These games will of course still need to be purchased to play.
It is suspected that this service is available in the UK as well as the US, though not confirmed. A Microsoft spokesperson was unavailable for comment at the time of writing.
Source: Major Nelson.