Apple's Xcode development tools are available through the Mac App Store, but some people have run into some problems when installing from the store, including hangs during installation and data loss.
Apple has made version 3.2.2 of their Xcode developer tools available for download.
Xcode to be used for iPhone development; iPhone simulator included
Build native iPhone applications in Leopard with Xcode 3.0
Apple Xcode 2.4.1 released: Stability, security fixes
Xcode 2.4 released: Stability and performance enhancements
Xcode 2.3 released
Apple has laid down the law on new app development. All new apps must be created with the latest Xcode software, and must conform to the design language of iOS 7.
Utilities Updates: Firefox 1.5rc2; Apple Xcode 2.2; StuffIt Expander 10.0.1; MimMac 1.7; more
XCode 2.1 released -- with support for creating Universal Binaries (Intel-compatible Mac OS X applications)