I wanted to sign up with MSD Adcenter (Google AdWords alternative) recently, but all the promo codes seemed to have expired. I did some digging, as was able to find a working promo code that gives you a $50 MSN Acenter Credit for new sign-ups.
Sign up here, use promo code: AD4511
Adcenter Promo Code valid only through April 30, 2007 (midnight Pacific Time), while supplies last. Promotional credit is valid only for new advertisers opening a new account with Microsoft adCenter. This does not include the one-time, non-refundable $5 activation fee required to open a new adCenter account.
A quick tip: MSN Adcenter allows you to have up to 3 accounts PER credit card. So, even if you have one account, you should consider opening up another for a free $45 (after $5 sign-up fee).