DCDev

BootDreams 106b - A utility that creates self booting discs
DC Dev ISO r4 - I know it's a bit outdated (2008) but it's a must for devs who want to get up and running quickly. It provides a complete environment to start creating content for the DC. Just burn the ISO to a disc using something like BootDreams and you're set! Really this is like all you need. :)
DCLoad Serial Kit - Used to load data onto a DC via a Coder's Cable (source files from AndrewK/Napalm)
DCLoad IP Kit - Used to load data onto a DC via a Broadband Adapter (source files from AndrewK/Napalm)
nullDC 1.0.4 - A great DC emulator (source files from nullDC Google Code)
Elf2Bin Console 1.0 - Super easy to use tool for converting .elf files to unscrambled .bin files ready for burning. B) Make sure to also download the neccessary .dll file Elf2Bin DLL 2.1

BOOB! - So much good info here!
KallistiOS - Learn all about the KOS OS which is used to build games for the DC, GBA and more.
Dreamcast Emulation Forum - Dang, such a good resource and community to learn stuff!