In app advertising can generate significant revenues for you. There are 2 scenarios of usage for your free app that can determine the in app ad revenues. Either your app can be mainstream or have a small niche user base. If you app is mainstream small amounts of usage can add up, and if you app has a small niche user base but is very “sticky”, then the revenues can also add up.
Here is a bunch of Ad Networks that you can consider, if you want to earn revenues through ads inside your apps –
http://www.admob.com – The 1000 pound gorilla in the space that was recently acquired by Google.
http://www.adwhirl.com – A multi-network advertising platform with a feature that allows you to switch between Admob, Mobclix, JumpTap, videoEgg, Millenial Media, Quattro Wireless and more.
http://www.google.com/ads/mobileapps – Google Adsense for mobile applications.
http://www.greystripe.com – Has a propritery method of displaying flash ads inside the iphone and recently uses highly immersive and interactive in-app ads.
http://www.medialets.com – Specializes in rich media advertising for mobile devices. They offer an analytics platform and has a framework for tracking user activity.
http://www.mobclix.com – Represents the largest mobile ad networks available for developers and publishers and supports over 20 different networks, and has analytics integration for developers.
http://www.quattrowireless.com – operates as a mobile adnetwork exchange as well as offering its own ads. Allows location based ad serving / targeting.
http://www.tapjoy.com – Operates as an ad network exchange and also has an iPhone SDK for selling virtual goods via Offerpal Media. Has a pay-per-install feature also.
http://www.ivdopia.com – Claims to be the first iPhone Video Ads platform.
http://www.videoegg.com – Another ad network exchange service. Has an interesting Ad Frame format which when tapped shows a full page ad.