IndustryBrains

I almost hate to give away this nugget for high quality CPC traffic on business sites. I have found the best conversion rates with some of the highest quality leads off IndustryBrains traffic. Their content network is one of the largest out there, and features sites such as BusinessWeek.com, InfoWorld, and the Motley Fool.

I recommend: IndustryBrains' network is organized by industry, making it easier to find placements for your ads. 

Business.com

They pair a great directory system with the ability to advertise on keyword searches done on the Business.com site and network. Their listings and search results are also featured on sites such as BusinessWeek.com, Forbes.com, the Wall Street Journal, and, are, of course, the owners of Work.com.

I recommend: Business.com still has a relatively low CPC for many listings, while giving you great access to business-only prospects. 

KnowledgeStorm

With great reach, KnowledgeStorm is one of the dominant forces in B2B technology online advertising. They work with both the search and content network, and include such giants as BusinessWeek.com, Forbes.com, ComputerWorld, and Inc.com.

I recommend: The only downside is that KnowledgeStorm sells on a CPL basis, so be prepared to spend the big bucks if you go with them.  Plus, their traffic comes from visitors to their solution centers within publisher’s websites, so you’re only getting access to the traffic which surfs to this network. 

Quigo

More of a content network than a search engine, per se, but still a good place to be found in the B2B marketplace. Their network includes a broad range of sites including: IDG network, Forbes.com, CIO, NetworkWorld, CNNMoney, and Time.com to name a few.

I recommend: Quigo is a recent addition to this marketplace, and features pay-per-click advertisements placed on the adSonar network, among others. 

Microsoft adCenter

Poor Microsoft for getting into the game late. Their traffic is worth a shot, but I’ve found limited success for B2B companies because the volume just isn’t there. But, they’re one of the big three, so trial and error is necessary.

I recommend: Microsoft's adCenter covers general PPC search marketing, but may be a good expansion past Google and Yahoo for the B2B company. 

Other B2B marketplaces

Here are a few other options for you if you’ve already expanded to those above. These are generally smaller networks or focused on a specific vertical. Either way, they’re better options for the B2B arena than Ask.com or other general 2nd tier search engines.

I recommend: GlobalSpec focuses on engineering and industrial B2B searches.  IT.com features vertical search for the Information Technology marketplace. The well known ThomasRegister provides another option for those businesses selling industrial products or services.  BitPipe/TechTarget is an enterprise IT search engine using TechTarget which features white papers and webinars from vendors.