It took quite awhile for "ad exchanges" to arise online. Years after the proliferation of ad networks ad exchanges came along to aggregate inventory and bring more "liquidity" to the market. They also aim to overcome market fragmentation. Indeed, there are more than 400 online ad networks and, depending on how one defines exchange, 5-10 online ad exchanges. 

A parallel trend is taking off in mobile, even before mobile ad networks have reached anything approaching maturity. Today mobile ad exchange Mobclix announced that it had developed the largest mobile ad exchange:

Through partnerships with over 20 leading ad networks, Mobclix provides the largest community of developers, advertisers, agencies and ad networks on one common platform for consolidated relationship management. As marketers leverage the Mobclix platform for precise audience targeting based on geographic, demographic and behavioral insights, developers use its analytics and advertising expertise to gain access to multiple networks in order to secure 100 percent fill rates and the highest eCPMs.

Mobile ad networks (partial list):

• AdMob
• AdModa
• 4Info
• Google/DoubleClick
• Greystripe 
• JumpTap
• Medio Systems
• Microsoft
• Millenial Media
• Mojiva
• MoVoxx
• Nokia Interactive (formerly enpocket)
• Quattro Wireless
• ThirdScreen/AOL 
• Video Egg
• Yahoo!

Mobile "exchanges," "mediators" and/or marketplaces (partial list): 

• Adfonic
• AdMarvel
• AdWhirl
• Mobclix
• Phonespots (Call Genie, but haven't heard anything in some time)
• Smaato
• Tapjoy 

Also some of the "networks" in the first list are starting to incorporate quasi-exchange features and third party inventory.