There is no such thing as a free lunch. I believe you should ALWAYS pay for your XBox Live subscription.
Some people with no morals have, however, been going to
http://www.bk.com/halo3/default.aspx, entering a load of blag details and receiving a free 48 hour trial of XBox Live.
It's only one trial per E-Mail address, so some people (and I'm not going to name names here) have been using the services of
http://www.guerrillamail.com/ so that they can stockpile a load of XBox Live trials and not have to pay for it again.
Honestly... some people.