I implemented Yubikey authentication for
RoundCube, a web-based email client. The
patch I created can be downloaded
here. This patch was created against revision 1611 of the project and should work fine against the most recent release on their site (v0.2-alpha).
After applying the patch, you need to edit 3 yubikey related variables in the config/main.inc.php file to enable Yubikey authentication and specify your API key & ID. Then, you can log in normally and edit your account settings to enable Yubikey authentication for your account and to specify your particular Yubikey's public ID. In this implementation, I am using the first 12 characters of your Yubikey's output as the public ID. Note: If Yubikey authentication is not enabled in main config file, the Yubikey-specific fields on the login and settings screens will not appear.
Two screenshots of the app after applying the patch are attached to this post.
I'd be thrilled to get any kind of feedback on how this works for you. Also, I'll be happy to answer any questions regarding implementing this patch. Hopefully, this patch will find its way into the RoundCube project trunk.
-D