What are the most useful third-party iPhone platforms?

Question

What really effective third-party framework / tools / projects exist there that people used and found great help in creating their iPhone apps? Bonus points if you talk about how it helped you in the real world.

I'll go first:

• cocos2d
• JSON.framework
• AQToolkit

EDIT: Changed This Community Wiki

Answer 1

I would add the following to support social networks:

• Twitter API: MGTwitterEngine
• Facebook Connect

Answer 2

• Three20

• ASIHTTPRequest

Answer 3

Objective Resource allows you to connect your iPhone application to a rails application.

Answer 4

While some of the frameworks listed are Mac-only, the question is "What Cocoa / Cocoa Touch Frameworks open source is?" has links to many quality third-party frameworks in the answers. Of these, I used Core Plot in our automated management software to make it easier to add scatter plots (of course, I'm a little biased, as one of the project participants).