The official Dreamcast development IRC channel is #dreamcastdev on irc.freenode.net
Free Development Tools
There are three free development libraries you can choose to start developing for the Dreamcast console.
He also released a pre-compiled version of the Code::Blocks IDE for Windows. Check it out!
Documents and Tutorials
- Configuring a development environment under Linux by henzenmann
- 2D Graphics Tutorial by BlackAura
- Introduction to SDL 2D Graphics by reaper2k2
- PowerVR Introduction by BlueCrab
Official Development Tools
A list of quite all independet/homebrew Dreamcast developers can be found here.