• AmigaOS 3.5 SDK can be obtained from Haage and Partners, and other Amiga retailers. NOTE: You don't strictly need this, as you can develop using the 3.9 NDK (see link below), but having the NDK's for specific versions of the OS makes it easier to catch accidental use of too new functionality if you wish to target older releases.
• AmigaOS 3.9 Downloads Contains the official FAQ, the Native Development Kit, and various updates
• AmigaOS4 SDK is available for free download from Hyperion TODO: Link
• AROS SDK is available for free download from AROS TODO: Direct link
• MorphOS SDK is available for free download from http://www.morphos-team.net/downloads.html

The following libraries are used so often that they are pretty much essential to compile a lot of Amiga source:

* ReqTools development archive