Update 01/03: The promotion has had a limit of 2000 users which apparently was reached (see the comments to this post)
I have not spent the $100 I got from the previous promotion when it is time to get on a new one. :-) This is the best deal from Microsoft AdCenter I have seen so far.
Use promo code DM-2-1106 to open a new account before January 15, 2007 and get $200 in free ad clicks. The offer valid for new customers only (this usually means the email you use to sign-up should not already be in their database) and $5 initiation fee will apply.