Here is a list of the leading ad networks @ Dec 2015:
Search PPC
Google Adwords
MSN Adcenter
Social Networks
Facebook
Twitter
Plenty of Fish
Reddit
LinkedIn
In-text Ad Networks
50onRed
Infolinks
Clicksor
Pay per view PPV
Traffic Vance
Lead Impact
Media Traffic
Clicksor
Pop Unders
Wiget Media
Adcash
AdsTerra
Banner Display Networks / Exchanges
Sitescout
Google Adwords Display / Doubleclick
Media.net
Advertising.com
Engage BDR (First Impression)
BuySellAds
Adroll
OpenX
Rubicon
Matomy Media
Mobile Advertising Networks
Google Adwords (admob)
Millenial Media (jumptap)
Inmobi
BuzzCity
Leadbolt
Phunware (Tapit)
Appflood
Native Advertising
Yahoo Gemini
Taboola
Outbrain
Rev Content
Retargeting
Adwords
Sitescout
Facebook
Adroll
I’m almost certain I’ve missed someone off that list but there’s plenty of volume on these.