HQ: New York
CEO: Brian O'Kelley
New Funding: $75 million
Backers: Technology Crossover Ventures, Venrock, Tribeca Venture Partners, others
AppNexus' rapid growth has been in the online advertising marketplace, where its customers include everyone from Microsoft Advertising Exchange to Technorati Media. It's now a key company with TCV, one of the country's most powerful tech-focused private equity firms and whose investments also include Facebook, Netflix and Exact Target.