What are the most useful third party iPhone frameworks?

Question

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

Answer 1

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).