Current licensing details can be found at:
http://www.chiark.greenend.org.uk/~sgtatham/putty/download.html

The terms of the license at the time of this files creation are listed
below.  It covers both putty.exe and psftp.exe.

The PuTTY executables and source code are distributed under the MIT licence,
which is similar in effect to the BSD licence. (This licence is Open Source
certified and complies with the Debian Free Software Guidelines.)

The precise licence text, as given in the About box and in the file LICENCE
in the source distribution, is as follows:

    PuTTY is copyright 1997-2010 Simon Tatham.

    Portions copyright Robert de Bath, Joris van Rantwijk, Delian Delchev,
Andreas Schultz, Jeroen Massar, Wez Furlong, Nicolas Barry, Justin Bradford,
Ben Harris, Malcolm Smith, Ahmad Khalifa, Markus Kuhn, Colin Watson, and
CORE SDI S.A.

    Permission is hereby granted, free of charge, to any person obtaining a
copy of this software and associated documentation files (the "Software"),
to deal in the Software without restriction, including without limitation
the rights to use, copy, modify, merge, publish, distribute, sublicense,
and/or sell copies of the Software, and to permit persons to whom the
Software is furnished to do so, subject to the following conditions:

    The above copyright notice and this permission notice shall be included
in all copies or substantial portions of the Software.

    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
SIMON TATHAM BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE. 

In particular, anybody (even companies) can use PuTTY without restriction
(even for commercial purposes) and owe nothing to me or anybody else. Also,
apart from having to maintain the copyright notice and the licence text in
derivative products, anybody (even companies) can adapt the PuTTY source
code into their own programs and products (even commercial products) and owe
nothing to me or anybody else. And, of course, there is no warranty and if
PuTTY causes you damage you're on your own, so don't use it if you're
unhappy with that.

In particular, note that the MIT licence is compatible with the GNU GPL. So
if you want to incorporate PuTTY or pieces of PuTTY into a GPL program,
there's no problem with that.