Wait a minute...
*looks up at the address bar*
Whew, for a second I thought I was on IGN's XBOX Live Arcade message board with this asking for free XBLA trials.
Why not just make a new account on your XBOX 360? Every new account made and tied into Live gets a free 30 days... just create a new passport with some e-mail address (I used a spare Yahoo! one when I set up a Japanese XBL account) and tie that in. Free and faster.
PS. I do have a 30 day card sitting at home but I am not there

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