If you are looking to move beyond playing simple LED games with your Arduino, I recommend starting with these tutorials on the ATMEL AVR chips. You will need to read/reference the datasheets for the chips also. Many of these tutorials were originally posted on the AVRFreak’s forum. The following are mandatory reading in my mind:
• Newbie’s Guide to AVR Timers
• Newbie’s Guide to AVR Interrupts
• Using the EEPROM in AVR-GCC
• Using the USART in AVR-GCC
• Interrupt Driven USART in AVR-GCC
• AVR-GCC and the PROGMEM Attribute
µC eXperiment 