Lots of great stuff at the blog of Tom Igoe, a physical interaction designer and new media artist. He points to a nice little .zip file of circuit schematic symbols in Illustrator format. But even more essential is his tutorial on how to generate graphs of sensor data with three lines of Wiring code. I could definitely see using this instead of MATLAB to create quick example images for presentations and the like. Plus, all the software you need is free and comes standard with OS X.