Here is a small collection of potentially useful and intriguing tools that can be used, for instance, to analyze toroids, perhaps like this Oktava ribbon mic toroid that we took apart.
Most of these programs are directed at ham radio geeks which is why they are mostly free or very cheap. BUT, all have uses in the audio recording domain. Want to measure noise? Just substitute your preamp and IO. Care to watch a histogram of an entire song as it plays? Use Winrad to make a waterfall display and fool it into thinking it is looking at 20 Hz to 20 KHz. I figured it out, and so might you. Want to know what radio station that microphone and phantom power supply cabling is tuned to? Antenna SW may give you some insight.
ELSIE Filter design program and several other useful tools by WB6BLD
FSM Field Strength Meter by VK1OD Measures noise levels of sound
cards Receivers and RF SNR.The site has several other tools and articles
WinRad the DSP backend of Software Defined / Direct Conversion
Radios by I2PHD http://www.winrad.org/winrad/index.html
EZNEC Antenna Modelling software by W7EL
5SPICE Circuit design and analysis
Mini Ring Core Calculator by DL5SWB For designing toroidal inductors
MIXW A Swiss army knife set for demodulating digital modes and
operating tools by UT2UZ
Source: James D Koger, N1IPP