I just need to put in a word for Kicad http://www.kicad-pcb.org . Unlike the software mentioned here, it's a FOSS project so it has no artificial limitations in its low-cost/free version, and it's being continuously improved on all platforms it runs on (Windows/Linux/Mac). The output (Gerber files) from Kicad is often directly accepted by inexpensive PCB fabrication shops.