FreePCB – An open source PCB editor for Windows.
Power Supply PCB Design – Lazar has put together several very informative articles on designing pcbs related to switch mode supplies as well as general power supply design techniques.
RF Cafe – RF Cafe is a unique portal of RF, microwave, wireless, and other engineering resources. Vast collection of equations, references and conversions for electrical, mechanical, physical, and mathematical fields. Links for vendors, app notes, test notes, ham radio, magazines, books, active forums, patent and communications standards, technical crossword puzzles, quizzes, and much more.
Lakeview Research – The developer’s resource for computer interfacing, especially USB, mass storage, serial (COM) ports, Ethernet and Internet communications for embedded systems, and parallel port.
Golden Gate Graphics – Printed Circuit Design and Manufacturing