If you like to use Cygwin and Mercurial together, like I do, it might be handy to know how to get your Mercurial commands from inside Cygwin to use PuTTY’s Pageant as its agent for SSH keys.  It’s pretty simple, using PuTTY’s Plink. Plink is the PuTTY tool designed for non-interactive ssh usage.

Just add an ssh entry to the [ui] section of your .hgrc file:

[ui]
ssh = "C:/Program Files/PuTTY/plink.exe"