Here's a handy class for applications needing a way to deal with mathematical expressions input by the user. It accepts an NSString consisting of a well-formed mathematical expression, evaluates it, and returns the result. It supports arbitrary named variables, so it is also perfect for graph plotting applications.

Yet Another JSON Library. YAJL is a small event-driven (SAX-style) JSON parser written in ANSI C, and a small validating JSON generator. YAJL is released under the BSD license.

ParseKit is a Mac OS X Framework written by Todd Ditchendorf in Objective-C 2.0 and released under the MIT Open Source License. ParseKit is suitable for use on Mac OS X Leopard, Snow Leopard or iPhone OS. ParseKit is an Objective-C implementation of the tools described in "Building Parsers with Java" by Steven John Metsker. ParseKit includes additional features beyond the designs from the book and also some changes to match common Cocoa/Objective-C conventions.

TBXML is a light-weight XML document parser written in Objective-C designed for use on Apple iPhone / iPod Touch devices. TBXML aims to provide the fastest possible XML parsing whilst utilising the fewest resources. This requirement for absolute efficiency is achieved at the expence of XML validation and modification. It is NOT possible to modify and generate valid XML from a TBXML object and NO validation is performed whatsoever whilst importing and parsing an XML document.