For all you electrical engineers (EE) out there, i have found this EEToolbox. It has calculators for:
PCBs (Trace width/current/resistance/impedance)
Passive (Resistor color bands, Inductance)
RF (Impedance, unit converter, Cross Talk)
General EE math (Algebra, geometry, Trigonometry, Derivatives, Integrals)
Graph paper and logs
All of these are essential (especially the Passive section) to any engineer as i have found in my PLTW Intro to Engineering courses in High School.