A number of CAD tools are helpful in day to day design even though the real design tool is the one the designer has between his ears. As the acronym suggests, CAD tool are intended to help and assist the designer in his task.
Spreadsheet tools are among the simplest and most available ones. A problem that can be solved using a spreadsheet should not be solved with a more complex tool. This is a matter of efficiency but also it requires to formalize the problem properly which is the first step of solving.
CAS tools such as XCAS or Eigenmath are helpful to perform symbolic calculations.
These tools such as Matlab or Scilab, but also most electrical simulators through behavioral description languages are helpful to define blocks specification and to check complex systems operation.
These tools, more or less derived from ancestor program SPICE are probably the most important tools for analog designers. A version such as LTSPICE IV is free, powerful and user friendly
These tools are intended to extract voltages, currents, impedance values etc... from a 3D description. My own EZMod3D is very efficient at that. It can read gds files. When layers physical properties, thickness and relative positions are defined, it can extract parasitic capacitance, resistance or both, but it can also perform thermal analysis and much more.