Welcome to Keyboard-to-Fretboard. This tool is designed to present an interactive visual representation of the relationship between the notes on a piano keyboard and where the same notes on the fretboard of a stringed instrument, such as a guitar, mandolin etc. This is one of a set of music and instrument related tools, along with Theo's Chord Generator.
To use the tool, select your instrument from the following list. The piano keyboard at the top will then change to reflect the playable range* of your chosen instrument, and the fretboard diagram will change to reflect the tuning and stringing of the instrument. You can then hover the keys on the keyboard, and you will be shown the position(s) at which you can play those same notes on your chosen instrument. If you wish to lock the fret diagram to a specific note, just click the keyboard rather than hovering (double clicking unlocks it again.)
Regarding the virtual piano keyboard, a few visual aids are provided: Middle C is displayed in yellow and the the A above Middle C is shown in blue (this is the international reference pitch, usually pitched at 440 Hz.)
There are a few caveats: accurately representing unique instrument features such as the 5-string banjo's truncated 5th string is rather hard to do, and/but it's taken for granted that if you play the instrument, you'll know how to interpret the information. Also, instruments such as the guitar are often notated in sheet music an octave above their actual sounding pitch. For this reason, there are two entries for these instruments, so that you can choose the one which best fits your purpose. This is done partly in order to make them fit better onto the treble clef (without excessive ledger lines), and also because music is often played on these instruments an octave below it is played on other instruments.
(*) The practically playable upper range may vary from instrument to instrument. 24 frets are shown, but in reality you may only get 21 or 22 frets on your neck unless you're playing an electric guitar. The minimum number of frets is probably 17, for short scale tenor banjo.
This tool is free software (released under the GNU AGPLv3) - the source code is bundled with the Chord Generator source, and can be downloaded here.