iPhone SDK 2.2.1 Leopard (10.5.4)
http://developer.apple.com/iphone/download.action?path=/iphone/iphone_sdk_for_iphone_os_2.2.1__9m2621a__final/iphone_sdk_for_iphone_os_2.2.19m2621afinal.dmg

iPhone SDK 3.0 (Xcode 3.1.3) Leopard (10.5.7)
http://developer.apple.com/iphone/download.action?path=/iphone/iphone_sdk_3.0__final/iphone_sdk_3.0__leopard__9m2736__final.dmg


iPhone SDK 3.0 (Xcode 3.2) Snow Leopard (10.6.0)
http://developer.apple.com/iphone/download.action?path=/iphone/iphone_sdk_3.0__final/iphone_sdk_3.0__snow_leopard__final.dmg

iPhone SDK 3.1 with Xcode 3.1.4 Leopard (10.5.7)
http://developer.apple.com/iphone/download.action?path=/iphone/iphone_sdk_3.1__final/iphone_sdk_3.1_with_xcode_3.1_final__leopard__9m2809.dmg

iPhone SDK 3.1 with XCode 3.2.1 for Snow Leopard (10.6.0)
http://developer.apple.com/iphone/download.action?path=/iphone/iphone_sdk_3.1__final/iphone_sdk_3.1_with_xcode_3.2_final__snow_leopard__10a432.dmg

iPhone SDK 3.1.2 with XCode 3.1.4 for Leopard (10.5.7)
http://developer.apple.com/iphone/download.action?path=/iphone/iphone_sdk_3.1.2__final/iphone_sdk_3.1.2_with_xcode_3.1.4__leopard__9m2809.dmg

iPhone SDK 3.1.2 with XCode 3.2.1 for Snow Leopard (10.6.0)
http://developer.apple.com/iphone/download.action?path=/iphone/iphone_sdk_3.1.2__final/iphone_sdk_3.1.2_with_xcode_3.2.1__snow_leopard__10m2003.dmg

iPhone SDK 3.1.3 with XCode 3.1.4 for Leopard (10.5.7)
http://developer.apple.com/ios/download.action?path=/iphone/iphone_sdk_3.1.3__final/iphone_sdk_3.1.3_with_xcode_3.1.4__leopard__9m2809a.dmg

iPhone SDK 3.1.3 with XCode 3.2.1 for Snow Leopard (10.6.0)
http://developer.apple.com/ios/download.action?path=/iphone/iphone_sdk_3.1.3__final/iphone_sdk_3.1.3_with_xcode_3.2.1__snow_leopard__10m2003a.dmg

iPhone SDK 3.2 Final with Xcode 3.2.2 for Snow Leopard (10.6.0)
http://developer.apple.com/ios/download.action?path=/iphone/iphone_sdk_3.2__final/xcode_3.2.2_and_iphone_sdk_3.2_final.dmg

Xcode 3.2.3 and iPhone SDK 4 GM seed for Snow Leopard (10.6.2)
http://developer.apple.com/ios/download.action?path=/iphone/iphone_sdk_4_gm_seed/xcode_3.2.3_and_iphone_sdk_4_gm_seed.dmg

Xcode 3.2.3 and iPhone SDK 4 Final for Snow Leopard (10.6.2)
http://developer.apple.com/ios/download.action?path=/iphone/iphone_sdk_4__final/xcode_3.2.3_and_iphone_sdk_4__final.dmg

Xcode 3.2.3 and iOS SDK 4.0.1 for Snow Leopard (10.6.4)
http://developer.apple.com/ios/download.action?path=/ios/ios_sdk_4.0.1__final/xcode_3.2.3_and_ios_sdk_4.0.1.dmg

Xcode 3.2.3 and iOS SDK 4.0.2 for Snow Leopard (10.6.4)
http://developer.apple.com/ios/download.action?path=/ios/ios_sdk_4.0.2__final/xcode_3.2.3_and_ios_sdk_4.0.2.dmg

Xcode 3.2.4 and iOS SDK 4.1 for Snow Leopard (10.6.4)
http://developer.apple.com/ios/download.action?path=/ios/ios_sdk_4.1__final/xcode_3.2.4_and_ios_sdk_4.1.dmg

Xcode 3.2.5 and iOS SDK 4.2 GM for Snow Leopard (10.6.4)
http://developer.apple.com/ios/download.action?path=/ios/ios_sdk_4.2_gm_seed/xcode_3.2.5_and_ios_sdk_4.2_gm_seed.dmg

Xcode 3.2.5 and iOS SDK 4.2 for Snow Leopard (10.6.4)
http://developer.apple.com/ios/download.action?path=/ios/ios_sdk_4.2__final/xcode_3.2.5_and_ios_sdk_4.2_final.dmg

Xcode 3.2.6 and iOS SDK 4.3 GM Seed for Snow Leopard (10.6.6)
http://developer.apple.com/devcenter/download.action?path=/ios/ios_sdk_4.3_gm_seed/xcode_3.2.6_and_ios_sdk_4.3_gm_seed.dmg

Xcode 3.2.6 and iOS SDK 4.3 for Snow Leopard (10.6.6)
http://adcdownload.apple.com/Developer_Tools/xcode_3.2.6_and_ios_sdk_4.3__final/xcode_3.2.6_and_ios_sdk_4.3__final.dmg

Xcode 3.2.6 and iOS SDK 4.3.1 for Snow Leopard (10.6.6)
http://adcdownload.apple.com/Developer_Tools/xcode_3.2.6_and_ios_sdk_4.3__final/xcode_3.2.6_and_ios_sdk_4.3.dmg

Xcode 4 and iOS SDK 4.3 for Snow Leopard (10.6.6)
http://developer.apple.com/devcenter/download.action?path=/Developer_Tools/xcode_4_and_ios_sdk_4.3__final/xcode_4_and_ios_sdk_4.3__final.dmg


Xcode 4.0.1 and iOS SDK 4.3.1 for Snow Leopard (10.6.6)
http://developer.apple.com/devcenter/download.action?path=/Developer_Tools/xcode_4.0.1_and_ios_sdk_4.3/xcode_4.0.1_and_ios_sdk_4.3.dmg


Xcode 4.0.2 and iOS SDK 4.3.2 for Snow Leopard (10.6.6)
http://developer.apple.com/devcenter/download.action?path=/Developer_Tools/xcode_4.0.2_and_ios_sdk_4.3/xcode_4.0.2_and_ios_sdk_4.3.dmg

Xcode 4.1 for Snow Leopard (10.6.6)
https://developer.apple.com/downloads/download.action?path=Developer_Tools/xcode_4.1_for_snow_leopard_21110/xcode_4.1_for_snow_leopard.dmg

Xcode 4.1 for Lion (10.7)
https://developer.apple.com/downloads/download.action?path=Developer_Tools/xcode_4.1_for_lion/xcode_4.1_for_lion.dmg

Xcode 4.2 and iOS SDK 5.0 for Snow Leopard (10.6.6)
https://developer.apple.com/downloads/download.action?path=Developer_Tools/xcode_4.2_for_snow_leopard/xcode_4.2_for_snow_leopard.dmg

https://developer.apple.com/devcenter/download.action?path=/Developer_Tools/xcode_4.2_with_ios_5_sdk/xcode_4.2_and_ios_5_sdk_for_snow_leopard.dmg

Xcode 4.2 and iOS SDK 5.0 for Lion (10.7)
https://developer.apple.com/downloads/download.action?path=Developer_Tools/xcode_4.2_for_lion_21179/xcode_4.2_for_lion.dmg

http://developer.apple.com/devcenter/download.action?path=/Developer_Tools/xcode_4.2_with_ios_5_sdk/xcode_4.2_and_ios_5_sdk_for_lion.dmg

Xcode 4.2.1 and iOS SDK 5.0 for Lion (10.7)
http://developer.apple.com/downloads/download.action?path=Developer_Tools/xcode_4.2.1_for_lion/xcode_4.2.1_for_lion.dmg

Xcode 4.3 and iOS SDK 5.0 for Lion (10.7.2)
Note: For Xcode 4.3, you need also to install the optional components such as command line tools, and previous iOS Simulators from Xcode’s Download’s preferences
http://adcdownload.apple.com/Developer_Tools/xcode_4.3_for_lion/xcode_4.3_for_lion.dmg

Xcode 4.3.1 and iOS SDK 5.1 for Lion (10.7.3)
http://adcdownload.apple.com/Developer_Tools/xcode_4.3.1_for_lion/xcode_4.3.1_for_lion.dmg