Here are a few links to tools & libraries that I find useful.

Arduino

• Arduino IDE Download
• Adding support for other chips
• Maker Pro

Atmel ATtiny Datasheets

• ATtiny13A
• ATtiny25/45/85
• ATtiny2313

Atmel Mega Datasheets

• ATmega8
• ATMega48/88/168/328

Eagle

• Eagle is the tool I use to design PCBs
  Eagle Download Site
  Adafruit Eagle library
  Sparkfun Eagle libraries
  Eagle library for headers
  Eagle library for SMD headers

Other Chips

• NXP PCF8563P – Real Time Clock (RTC)