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
Credits go to Cédric Luthi for telling us the correct url above
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)
https://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