1 - Engineering References:

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The reference that launched a career. Steve Circia and his column from Byte magazine back in the '80s spurred along my hobby interest in embedded processor systems. Now a full blow magazine, a subscription (in my opinion) is the best investment one can make. Selected articles are also available online.
efunda is a site dedicated to engineering fundamentals. It is a worthwhile collection of referencen matrials including materials properties, stress strain caluculations, mathematics, etc. All in all, very cool!
Need to find a quick starting point to some Verilog source code? This is the Place, OpenCores.org has pretty much every thing a a guy needs that doesn't fall into the catergory of IP's that are so big they've got to be paid for.
The University of Missouri sponsors this site that contains an easy to use strip-line calculator. Click on the form you are interested in, enter your variables and Viola!
This site hosts a trace width calculator based on IPC data for free air and buried insulated traces, lengths and current carrying ampacities.