Bing/Microsoft & Yahoo Network can be interesting if you target an international audience:

• In the United States, 172 million unique searchers on the Bing Network who represent 5.6 billion monthly searches, accounting for 30% of the search market
• In Europe, 142 million unique searchers on the Bing Network who represent 3 billion monthly searches, accounting for 6.7% of the search market
• In Australia, 9 million unique searchers on Bing who represent 133 million monthly searches, accounting for 10.7% of the search market