Developer Wiki
The official Hiptop Developer Wiki is an incredible resource for information on the API, SDK, Resources, Techn Notes, Documentation, Hints and a place to discuss your development with others.

Hiptop Developer FAQ
covers questions regarding application approval and deployment.

Official Developer Forum
Talk with others about developing for the Hiptop and Sidekick (requires application acceptance).

SDK
Download the official Danger SDK, Resource Editor, library files for the variety of manufacturers (StarHub, E-Plus, Edge, Fido, T-Mobile, etc) , Guidelines and other important files.

SKDR.net
Although SKDR is an unofficial developer site, it is chock full of essential resources. Not only is there a public forum and Wiki, you can also freely submit and download hiptop applications submitted by others.