Here are a few links to tools & libraries that I find useful.
Arduino
Atmel ATtiny Datasheets
Atmel Mega Datasheets
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)