MOSS 2007 and WSS 3.0 SDK's released
The MOSS 2007 SDK includes the WSS 3.0 SDK, so you don't need to download both if you are going to download the MOSS 2007 SDK.
Here are the links:
SharePoint Server 2007 SDK: Software Development Kit and Enterprise Content Management Starter Kit
Windows SharePoint Services 3.0: Software Development Kit (SDK)