Here is the open source iphone app list in alphabetical order:

1. ABC 123 – Sequence memorization game. Utilizes Cocos2D. (itunes link) (source code)

2. Artifice – Strategy game where you try to get to the other side by moving boxes out of the way. Utilizes Cocos2D. (itunes link) (source code)

3. Battle For Wesnoth – Fantasy themed turn based tactical RPG game available on several platforms and now the iPhone/iPad. (itunes link) (source code)

4. Colloquy – Conversion of the most popular Mac IRC client to the iPhone.(itunes link) (source code)

5. Countitout - A generic counting app. (itunes link) (source code)

6. Diceshaker - Dice rolling simulator designed for role-playing game enthusiasts. (itunes link) (source code)

7. Doom Classic - Classic 3D first person shooter. (itunes link) (source code) (build instructions)

8. Ecological Footprint - Calculate, display, and record your ecological footprint. (itunes link) (source code)

9. Fosdem - Calendar app for the Fosdem open source conference. (itunes link) (source code)

10. Freshbooks – Open Source iPhone app that enables usage of Freshbooks web invoicing software from your iPhone. (itunes link) (source code)

11. Gorillas – Classic Worms/iShoot turn based shooter type game converted to iPhone from basic. Utilizes Cocos2D. (itunes link) (source code).

12. iStrobe - Turns the iPhone 4 flash into a highly configurable strobe light. (itunes link) (source code)

13. Last.fm – Software that enables usage of the Last.fm platform for personal radio stations. (itunes link) (source code)

14. Mobilesynth - A monophonic synthesizer designed for live performance.(itunes link) (source code)

15. Molecules – Allows you to view 3D models of molecules and manipulate them through touch. (itunes link) (source code)

16. Mover – Allows you to transfer stuff from one iPhone to another by “flicking” it to the other device. (itunes link) (source code)

17. Natsulion - A basic twitter client converted from mac. (itunes link) (source code)

18. NevoChess – A Xiangqi game. (itunes link) (source code)

19. NowPlaying – Allows you to check local theater listings, and check rotten tomato ratings. (itunes link) (source code)

20. Packlog – Backpack journal client. (itunes link) (source code)

21. PlainNote - Simple Open Source Notepad. (itunes link) (source code)

22. PocketFlix – Find movies, and manage your Netflix information. (itunes link) (source code)

23. Puff Puff – Beautiful underwater 2D game utilizing Cocos2D and Openfeint featuring unique physics based gameplay. (itunes link) (source code)

24. reMail – E-mail client featuring ultra-fast search. Removed from app store, but source made available. (source code)

25. RobotFindsKitten – Port of a very silly “classic” ASCII game. (itunes link) (source code)

26. Sci-15 HPCalc – Calculator app based on classic scientific HP-Calculator.(itunes link) (source code)

27. SpaceBubble – Space game featuring core graphics, and accelerometer usage. (itunes link) (source code)

28. Star3Map – Augmented reality star and planet charting application.(itunes link) (source code)

29. Stockfish Chess – A chess game featuring extremely effective AI .(itunes link) (source code)

30. Task Coach – Personal to-do list and task manager. (itunes link) (source code)

31. Tux Rider – iPhone port of the extremely popular, and beautiful 3D Tux Racer game. (itunes link) (source code)

32. Tweejump – Platform jumping game inspired by Icy Tower. Utilizes Cocos2D. (itunes link) (source code)

33. Tweetee – Enhanced version of the Natsulion Twitter Client. (itunes link) (source code)

34. Tweetero – Basic twitter client with image uploading. (itunes link) (source code)

35. Twitterfon – Super-fast intuitive twitter client. (itunes link) (source code)

36. ViralFire – Unique game where you are a dodging blood cell. (itunes link) (source code)

37. Wikihow – A reader app for the popular how to wiki site. (itunes link) (source code)

38. Wolfenstein 3D Classic Platinum – If you haven’t heard of Wolfenstein post below so we can say a prayer for you. (itunes link) (source code)

39. WordPress – Client for managing WordPress blogs. Also has an iPad version. (itunes link) (source code)

40. Xpilot – Simple massively multiplayer arcade shooter from the early days of the internet. (itunes link) (source code)

41. YourRights – Pocket database containing a summary of your legal rights. (itunes link) (source code)

42. ZBar – A barcode reader open source iphone app. (itunes link) (source code)

Check them out if you are working on something similar or think they might have a feature you could use there’s no reason to re-invent the wheel.  Always make sure you understand the licenses as many of these open source iPhone apps are GPL and require that you open source your app.