Travel Ad Network (TAN) is the leading vertical ad network focused exclusively on travel. It reaches users across 50 websites including Lonely Planet, Kayak, RandMcNally, Skybus Airlines, Groople, TravelHero, BootsNAll, AreaGuides, HotelsByCity, EuropeForVisitors, AirGorilla, and many others. Top advertisers with TAN include American Express, Vacations to Go, Best Western, Netherlands Tourism, Tourism Australia, etc.
Imran Khan’s report also has the following chart, which I found useful: Brand advertising budgets are moving to the web aggressively, and this chart is a good synthesis of the trend.