JOSM was originally designed and coded by Immanuel Scholz,
and is now maintained by the OpenStreetMap community.

The current JOSM maintainer is Dirk Stöcker.
A lot of administration work is done by Paul Hartmann and Vincent Privat.

Major code contributions from (in alphabetical order):

Stefan Breunig
David Earl
Gabriel Ebner
Dave Hansen
Paul Hartmann
Karl Guggisberg
Matthias Julius
Alexei Kasatkin
Jiri Klement
Ulf Lamping
Simon Legner
Raphael Mack
Upliner Mikhalych
Vincent Privat
Frederik Ramm
Dirk Stöcker

Many minor contributions and patches by others; see SVN history
at http://josm.openstreetmap.de/svn/ for details. Use
"josm-dev AT openstreetmap.org" to contact still active authors.

Copyright rests with the contributors.

The jpeg metadata extraction code is from Drew Noakes
(http://code.google.com/p/metadata-extractor/) and licensed
with Apache license version 2.0.

Most icons have been nicked from GNOME and the GIMP, both
under GPL. Some are originally LGPL but redistributed here under 
GPL. The world image is from onearth.pl.

The Bzip2 code is from Keiron Liddle (Apache project) and licensed
with Apache license version 2.0.

The signpost code (http://code.google.com/p/oauth-signpost/)
is from Matthias Käppler and licensed with the Apache License 2.0.

The NTv2 transformation code (http://jgridshift.sourceforge.net/)
is from Peter Yuill and licensed with LGPL.

The MultiSplitPane is from Hans Muller and licensed with LGPL
(https://today.java.net/pub/a/today/2006/03/23/multi-split-pane.html).

The Diff code (http://www.bmsi.com/java/#diff) 
is from Stuart D. Gathman and licensed with GPL.

The Ellipsoid code (http://www.i3s.unice.fr/~johan/gps/)
is from Johan Montagnat and licensed with GPL.

The opening hour validation uses code from opening_hour.js
(https://github.com/ypid/opening_hours.js) which is licensed
with the New (2-clause) BSD license.