coolkey is a pkcs#11 smartcard library that can be used with a US
Government CAC (Common Access Card), in conjunction with pcsc-lite.
It is most commonly used by pkcs#11 aware web browsers or email clients, to
use a CAC's client certificate for authentication or signing.

coolkey is commonly used with the DoD Configuration plugin for Firefox
and Thunderbird, at https://addons.mozilla.org/en-US/thunderbird/addon/3182.

This requires mozilla-nss and pcsc-lite (in that order).