For the US market, second tier search engines are search engines beyond Google, Bing and Yahoo (Yahoo results are provided by Bing).
According to Comscore, second tier search engines represented in October 2012 only 5% of US search queries behind Google (66,9%), Bing (16) and Yahoo (12,2).
"Major" US 2nd tier search engines are Ask network (3,2%) and AOL (1,8). Others are Excite, Altavista, Business.com, ...
Strictly speaking a 2nd tier search engine differs from a 2nd tier PPC network as often a 2nd tier search engine does not sell directly search ads to advertisers but is integrated in a network offer.