What really super useful third party frameworks / toolkits / projects are out there that people have used and have found to be a huge help in building their iPhone apps? Bonus points if you include a story about how it helped you on a real world project.
I'll go first:
EDIT: turned this community wiki
I would add the following for social networking support:
While some of the frameworks listed are Mac-only, the question "What open source Cocoa/Cocoa Touch Frameworks are out there?" has links to many quality third-party frameworks in the answers. Of those, I've used Core Plot within our robotic control software to simplify the addition of scatter plots (of course, I'm a little biased, being one of the contributors to the project).
Objective Resource lets you connect an iPhone app to a rails app.