In case you are considering trying Microsoft adCenter, they are now offering $100 to new customers. You must start your account with at least $5.00. This might be a good way to give adCenter a try.

adCenter Signup
Coupon Code : BNR-100-1006 (Expires: 01/15/07)

(we do not make anything off this referral, just want to help the search engine marketing community)