If you don't use cygwin or pageant, you don't need charade. If you're happy with plink, you don't need charade. (I love putty, but I prefer openssh to plink.)
Background: I tolerate Windows XP. I actually quite like it. But it needs putty, and it needs cygwin to be usable. I used to maintain two separate sets of ssh keys: one for putty, which I kept in pageant, and another for cygwin's openssh, which I kept in ssh-agent/keychain.
Eventually I got fed up maintaining two key pairs and charade was born.
Charade just pretends to be ssh-agent on one side and putty on the other. It's a shim between openssh and pageant.
It aspires to be a drop-in replacement for ssh-agent, and that's how I use it atm. It works for me. It probably won't work for you.
I stole large swaths of code from (i) Simon Tatham's putty (mostly putty-src/windows/winpgntc.c) and (ii) openssh's (and Tatu Ylonen's and Marcus Friedl's) ssh-agent.c.
Also check out cuviper's ssh-pageant, which does something similar.
Russell Davis has a clever way of setting environment variables at boot so that processes launched outside your shells can also use your ssh keys.
Required cygwin packages
Clone the repository
git clone git://github.com/wesleyd/charade.git
Build and install charade
make && make install
Stop running charade or ssh-agent processes
killall charade # Only if upgrading killall ssh-agent # Only if not upgrading
Remove existing keychain files to clear references to ssh-agent
rm -rf ~/.keychain
Add these two lines to ~/.bash_profile:
keychain -q -Q . ~/.keychain/`hostname`-sh
Logout, launch a new shell or putty and try it out
If you have a key in pageant, you should be able to go where it points.
It just forwards ssh messages, so surprising (to me!) things Just Work, like adding keys with ssh-add instead of through the pageant gui.
Wesley Darlington, December 2009.