MY FAVORITE TOOLS:
BestPair - I wrote this myself and I use it almost every day. Absolutely the very best way to determine the best pair of standard value resistors for a voltage divider. More info here -->
TK! Solver - I use this for all system modeling problems and to help develop software algorithms. I model everything from circuits to lamps to soup and nuts. More info here -->
Derive - Like MathCAD and others. Companion tool for software development. Great for transforming equations, algebra, trig, calculus. I also use it to plot functions, Good for combining and simplifying equations, calculus, generating Taylor Series & etc. More info here -->
Table Curve - We often need a mathematical function to describe analytical test data for software development. This is the best curve fitter I know of. More info here -->
Cheers, Tim