I've been playing with my Arduino this weekend, and I'm reallllly digging it. I did quite a bit with my Make Controller a couple months ago, and had a lot of fun, but this Arduino is even cooler. Really easy to setup, with an interface very similar to the Processing interface. Really excellent examples and documentation, and shows how important it is to have really well written examples and sample programs to get you going.
A serial communication program for accessing a microcontroller-based data-acquisition system
POSIX Threads Programming
Arduino - Smoothing analog input
ASCIITable - Demonstrates the advanced serial printing functions by generating a table of characters and their ASCII values in decimal, hexadecimal, octal, and binary.
Graph with Processing
Wiring - Similar to Arduino
Arduino grows up and learns to talk! by Lady Ada
Serial Programming Guide for POSIX Operating Systems
C++ Sockets Library