Mine is downright primitive compared to all of the ones the rest of you are posting, being a command line program in C that I originally wrote around 1986, probably for THINK C on my Mac Plus.

Here is the source code [1], only updated to compile cleanly with a modern C compiler and not to depend on any libraries or functions that were specific to THINK C, plus one bug fix and some reformatting.

Here are some usage examples:

  $ ./chord
  c
  C:  C E G
  <3>|-----|-----|-<5>-|-----|-----|-----|-----|-<1>-|-----|-----|-----|-<3>-|
     |-<1>-|-----|-----|-----|-<3>-|-----|-----|-<5>-|-----|-----|-----|-----|
  <5>|-----|-----|-----|-----|-<1>-|-----|-----|-----|-<3>-|-----|-----|-<5>-|
     |-----|-<3>-|-----|-----|-<5>-|-----|-----|-----|-----|-<1>-|-----|-----|
     |-----|-----|-<1>-|-----|-----|-----|-<3>-|-----|-----|-<5>-|-----|-----|
  <3>|-----|-----|-<5>-|-----|-----|-----|-----|-<1>-|-----|-----|-----|-<3>-|
  c7
  C7:  C E G A#
  <3>|-----|-----|-<5>-|-----|-----|-<7>-|-----|-<1>-|-----|-----|-----|-<3>-|
     |-<1>-|-----|-----|-----|-<3>-|-----|-----|-<5>-|-----|-----|-<7>-|-----|
  <5>|-----|-----|-<7>-|-----|-<1>-|-----|-----|-----|-<3>-|-----|-----|-<5>-|
     |-----|-<3>-|-----|-----|-<5>-|-----|-----|-<7>-|-----|-<1>-|-----|-----|
     |-<7>-|-----|-<1>-|-----|-----|-----|-<3>-|-----|-----|-<5>-|-----|-----|
  <3>|-----|-----|-<5>-|-----|-----|-<7>-|-----|-<1>-|-----|-----|-----|-<3>-|
  c#m
  C#m:  C# E G#
  <3>|-----|-----|-----|-<5>-|-----|-----|-----|-----|-<1>-|-----|-----|-<3>-|
     |-----|-<1>-|-----|-----|-<3>-|-----|-----|-----|-<5>-|-----|-----|-----|
     |-<5>-|-----|-----|-----|-----|-<1>-|-----|-----|-<3>-|-----|-----|-----|
     |-----|-<3>-|-----|-----|-----|-<5>-|-----|-----|-----|-----|-<1>-|-----|
     |-----|-----|-----|-<1>-|-----|-----|-<3>-|-----|-----|-----|-<5>-|-----|
  <3>|-----|-----|-----|-<5>-|-----|-----|-----|-----|-<1>-|-----|-----|-<3>-|

You simply type chord names, and it shows you what notes are in the chord and where you can find them on the guitar fretboard. The first example is a C chord. It tells you the notes you want are C E G. In the diagram a <1> means that is a place you can play the root of the chord, a <3> is the third, a <5> is the fifth. For the C chord, then, we see that on open first string would give us <3>, the third fret <5>, the root is on the 8th fret, and the <3> is also on the 12th fret.

Chord names are the root note (with # for sharp or b for flat), and modifiers 'm' for minor, '6', '7', or '9' for a 6th, 7th, or 9th chord. (I have no idea why I did not include diminished chords).

If a line starts with '-' it is a command to change settings. Commands are "-cN" to set a capo on the Nth fret

Here are the commands. "-cN" for a capo on the Nth fret. "-t<notes>" to set the tuning for each string, where <notes> is a list of 6 note names (which can each be followed by # or b). "-Td" or "-TD" for D tuning. "-sN" to transpose by N half-steps (N can be negative). "-n" to switch to a narrower display and "-w" to switch back to normal. Narrow display looks like this:

  C:  C E G
   3 |---|---|-5-|---|---|---|---|-1-|---|---|---|-3-|---|---|-5-|---|
     |-1-|---|---|---|-3-|---|---|-5-|---|---|---|---|-1-|---|---|---|
   5 |---|---|---|---|-1-|---|---|---|-3-|---|---|-5-|---|---|---|---|
     |---|-3-|---|---|-5-|---|---|---|---|-1-|---|---|---|-3-|---|---|
     |---|---|-1-|---|---|---|-3-|---|---|-5-|---|---|---|---|-1-|---|
   3 |---|---|-5-|---|---|---|---|-1-|---|---|---|-3-|---|---|-5-|---|

It's always amusing to look at one's old code. Much of it seems like it was written by someone else. For instance, I see a lot more spaces around punctuation than I would use today. I think that is probably because the editor I used back then did not have syntax highlighting, so extra space was needed to make sure you didn't confuse consecutive tokens.

[1] https://pastebin.com/0rZnaVgu