Sign Up at Yahoo/Adcenter to receive free clicks on all Yahoo and Bing search properties.
Yahoo! Search Marketing claims to have a traffic volume of 2.5 billion searches every month with 50 million unique visitors a day. From my experience the quality of paid search traffic from Yahoo is significantly different than traffic from Google adwords, so you should plan your bid strategy accordingly.
Yahoo/Bing sponsored search also offers a toll free phone number 866-747-7327 to answer questions about their online advertising services. I have tried to call them in the past, and it may take several tries but eventually you will be able to get a Yahoo customer service representative on the telephone.
Bing ads promo code for $25 free clicks USCJ21 is good for any new sponsored search signup.
Yahoo may have multiple promotional codes with a lower discounts. Use promo code USCJ21 to get a full $25 credit. Make sure you use this promo link only.
As bonus Yahoo credits are spotted please add the YSM codes in the comments.