Spenvis - Space environment modeling
Microcosm Books - Extensive selection of astronautics books including Wertz's, "Space Mission Analysis and Design"
VariCAD - 3D/2D CAD solution for mechanical engineering which unlike a lot of other CAD packages is affordable.
CadSoft Online - EAGLE Layout Editor - An inexpensive schematic capture and printed circuit board design package
ViewMate - Free Gerber Viewer
Parts and Materials
McMaster Carr - Parts, tools and just about anything you can think of
DigiKey - The source for electronic parts when in a hurry
PCB Express - Fast inexpensive printed circuit boards
eMachineShop - A great on-line machine shop (Download their free software, draw your part and click to order)
MfgQuote - Is a good source for getting quotes from machine shops
Education and Rocketry
Federation of Galaxy Explorers - Always looking for new volunteers