These are a few links that I find very useful:
www.researchindex.com : This is a massive search index of a lot a research papers. All available in either Postscript or PDF format. You want to look up something technical, do it here.
www.cs.wisc.edu/~ghost/ : This is the home of Ghostscript - a free PostScript interpreter. Download Ghostscript and GSView if you want to view Postscript (*.ps) documents, and convert them to Acrobat (*.pdf). Install a PS printer on your FILE: port and you can make your pdf's!
www.octave.org : Octave is a GNU replacement for MATLAB. Like all things GNU it is free. And its got (almost) 100% compatibility with MATLAB. It doesn't have the same level of graphics support, but its improving. And its great for people who cannot afford a MATLAB license.
http://pcb.cadence.com/Product/Simulation/PSpice/ : This is where you can get the Student Version of PSpice. Its fully functional except for a much smaller parts library. But remember you can always make a custom part.
www.nonags.com :This is the premier site for absolutely free software. (No nags!!). Look for only the 6 ducks and occasionally 5 ducks.
More coming!!