A configurable LaTeX template for formal letters,
using the scrlttr2 class of KOMA-Script.
You can move sender.lco and signature.jpeg to ~/texmf/tex/latex/
to avoid having to copy them for every letter.
Both files will be overridden by versions in the local directory,
if present.
This template is a pretty basic application of the scrlttr2 class.
I added a scanned image of the signature and
exports metadata to the resulting PDF
using tipps from Markus Kohm
on the de.comp.text.tex newsgroup and
on komascript.de, respectively.
Licensed under the terms of the CC0 1.0 Universal (CC0 1.0) Public Domain Dedication license and thus effectively part of the public domain.