General Resources:

• The Raspberry Pi Foundation, source of all things Pi
• Download site for raspberry Pi disk images
• Raspberry Pi quick start

Hardware vendors:

• The Pi Hut a great range of Pi and general electronics and robotics parts
• PiMoRoNi a great range of Pi and general electronics and robotics parts
• Mod My Pi a great range of Pi cases and other accessories
• adafruit.com run by the charismatic “Lady Ada”, a good source of all sorts of electronic hackery for your Pi
• CJE Micros a source of connectors, expansion boards and the like
• Proto-PIC UK dealer for various bits including some adafruit stuff
• HobbyTronics general electronics dealer with a raspberry Pi section
• PiBorg makers of the LEDBorg multi-colour LED, and an upcoming robot motor controller

Development Tools:

• MobaXterm ssh client with graphical X display capabilities, and much more
• Check unit test framework for C development
• The git version control system
• Yagarto, a GNU toolchain for cross-compiling ARM code on Windows
• MingW, a GNU toolchain for compiling code to run on Windows
• Cygwin, a GNU toolchain and unix-like environment for Windows

Tutorials:

• Baking Pi Operating Systems Development in ARM assembler

Software examples and projects:

• A port of the Arduino “Wiring” library to Raspberry Pi
• richcole: Adventures in Bare Metal Programming on the Raspberry Pi
• dwelch67: Lots of useful examples
• Some bare metal assembler examples not based on Baking Pi
• Some code based on Baking Pi, but with fixes and extensions
• raspi-os experiments with bare metal OS programming on Raspberry Pi
• Tart experiments with bare metal OS programming on Raspberry Pi
• LainOS-Pi A (attempt to?) port LainOS to Raspberry Pi
• Das U-Boot Ambitious multi-hardware boot loader
• A port of Xinu to ARM devices
• A few odd bits of low-level Raspberry Pi C code
• An attempt to port FreeRTOS to Raspberry Pi

Related, similar, or just plain interesting blogs:

• Raspberry PiPod | Experiences with the Raspberry Pi micro computer
• Gordons Projects
• MCU on Eclipse

Books and ebooks:

• Test Driven Development by example – Kent Beck
• Raspberry Pi User Guide – Eben Upton and Gareth Halfacree
• Haynes Raspberry Pi Manual – Eben Upton, Liz Upton and Gray Girling
• Raspberry Pi Projects – Andrew Robinson
• Raspberry Pi Assembly Language Beginners: Hands On Guide: 1 – Bruce Smith
• Raspberry Pi for Schools – Raspberry Shake
• Beginning Raspberry Pi Game Programming – Martin Bishop
• Raspberry Pi In Easy Steps – Mike McGrath
• Raspberry Pi – A Complete Beginner’s Guide – Bruce Smith
• Learn Raspberry Pi With Linux – Peter Membrey and David Hows
• Raspberry Pi For Dummies – Sean McManus and Mike Cook
• Getting Started with Raspberry Pi – Matt Richardson and Shawn Wallace
• Programming the Raspberry Pi: Getting Started with Python – Simon Monk