I had difficulty monetizing my app and gathered a lot of links I think might be useful to other people. Let's build the best list of ways to monetize apps! I'll update this list regularly with info from the comments.

Categories

Free To User

1) Ad Networks (Banner Ads)

2) Affiliate and CPA (Pay When User Installs Other App)

Cost To User

3) Paid Apps (Charge upfront for the app)

4) In-App-Purchase Approaches (Currency, Unlocking Features, Freemium)

5) Subscription (Recurring Data Updates, SAAS)

Misc

6) Facebook

7) Sponsors (Dedicated advertisers)

8) Email Lists (Alternative monetization method)

9) Merchandise (Works if the app has strong characters)

10) ?? Coming Soon ??

1. Ad Networks

Please share your experiences so this can become an ordered list with the best at the top.

1) TapJoy (http://developers.tapjoy.com/boost-your-revenue/)

2) AdMob (http://www.google.com/ads/admob/)

3) iAds (http://advertising.apple.com/developers/)

4) Adfonic (http://adfonic.com/mobile-advertising-services/publishers/)

5) Smaato (http://www.smaato.com/developers/)

6) InMobi (http://www.inmobi.com/developers/)

7) LeadBolt (http://www.leadbolt.com/developers.php)

8) Millenial Media (http://www.millennialmedia.com/monetize/)

9) MobYD (http://www.mobyd.com/Developers)

10) Trademob (http://www.trademob.com/)

11) Madvertise (http://madvertise.com/en/)

12) BuzzCity (http://www.buzzcity.com/f/pubdev)

13) AdModa (http://www.admoda.com/?page_id=26)

14) Mojiva (http://www.mojiva.com/mobile-advertising/monetize-your-mobil...)

15) Hunt Mobile Ads (http://www.huntmads.com/soy-un-developer)

16) Greystripe (http://www.greystripe.com/)

17) Madhouse (http://www.madhouse.cn/en/)

18) Jumptap (http://www.jumptap.com/developers/)

19) Mobile Theory (http://mobiletheory.com/developers/)

20) Microsoft Mobile Advertising (http://advertising.microsoft.com/mobile)

21) xAd (http://www.xad.com/publisher)

22) YP (AT&T) (http://corporate.yp.com/)

23) Tapgage (http://www.tapgage.com/)

24) Aditic (http://www.aditic.com/index.php/en/publishers.html)

25) iPhone Alliance (http://www.iphonealliance.com/)

26) Mobclix (http://www.mobclix.com/developers-faqs.html)

27) Vdopia (http://mobile.vdopia.com/)

28) Zumobi (http://www.zumobi.com/)

2. Affiliate / CPA Networks

1) MobPartner (http://www.mobpartner.com/en)

2) Sponsormob (http://www.sponsormob.com/en/publishers/)

3) Vizu (http://www.vizu.com/index.htm)

3. Paid Apps

1) The user pays upfront for the full app, usually at $0.99 or $1.99.

2) It has been shown that price and revenue do not always correlate precisely and you should experiment with various price points. Anchor it with the highest price point and put it on various levels of discount to test out sales.

4. In App Purchase (IAP) Approaches

1) Currency - Have a virtual currency used to buy boosters and perform other actions. The user can earn currency in-game or purchase more via IAP for faster results. Examples: Farmville's currency

2) Energy - The user gets a certain amount of actions per energy, and the energy replenishes at a certain rate. Items can be used to replenish the energy, and the items can be bought via currency funded via IAP.

3) Unlocking Features - Have premium features such as extra levels or assistance unlockable via IAP. Examples: Angry Birds's Eagle

4) Chips - Another take on currency, for casino type games. Examples: Zynga's Poker

5) Freemium - Similar to unlocking features, except the basic app is always free

6) Subscription - Covered in section 5, can be paid for via IAP or externally (such as Dropbox)

5. Subscription

1) Live Data Feed - Have the user pay a monthly fee in return for up to date data (Newspaper subscriptions, Manazine subscriptions, Stock Ticker Data)

2) Pay Per Data Usage - Have the user pay a recurring fee depending on how much they use (Examples: Dropbox)

3) Software As a Service - Pay for usage of the software as-you-go. I haven't seen a full SAAS mobile app yet but I think TaskRabbit is a close example.

6. Facebook

1)Facebook Approved Ad Providers(http://developers.facebook.com/adproviders/)

2)Facebook Currency Purchases (Example: FarmVille)

7. Sponsors

1) This involves finding third party sponsors and placing them individually in the app. Example: You have an app about tennis, and instead of placing ad-network ads you find a tennis racket retailer willing to pay you $X per month or $Y per click through to their site

8. Email Lists

1) Promote Paid Products - Get paid on traffic/conversion or promote your own paid products

2) Drive Traffic to a Website for Conversions/Ad revenue

9. Merchandise

1) Angry Birds has plushies and other merchandise

2) Imgur has shirts

3) Starcrafts turned a YouTube video series into t-shirts and plushies with memorable characters

4) Sell other people's merchandise for affiliate sales (Special version of category 2, Affiliate networks)Sources and Useful Articles:

http://mobithinking.com/mobile-ad-network-guide

http://www.seekomega.com/2010/12/a-comprehensive-comparison-...

http://stackoverflow.com/questions/5112939/what-is-the-best-...

http://onlineincometeacher.com/money/30-ways-your-website-ca...

http://www.business2community.com/blogging/how-to-monetize-y...