Unison is an excellent cross-platform file synchronization utility.
Unison fit the bill well. As with many Unix applications made cross-platform, sometimes getting the graphical user interface working properly on Windows can be troublesome. Below are guides I followed, and a few notes from my experience with the process. Many thanks to the original authors of these articles :)
Our setup was my brother's Windows XP desktop, my Kubuntu Linux desktop, and an Ubuntu LTS server. The server and my desktop are running Unixon 2.27.57. For my brother's Windows desktop we downloaded Unison 2.27.157 and GTK+ 2.17.7 rev A. You may find that another combination of software versions works for you, but we can confirm this combination is working. Follow the installation instructions on the Windows download page.
After getting things installed and we configured our profile to perform syncs over SSH. The first sync takes a while but subsequent syncs get shorter. One issue we ran into is that during the "Waiting for changes from server" step of the sync process SSH was timing out.
Fixing the issue was a quick change to sshd_config found in the OpenSSH FAQ. Found at
/etc/ssh/sshd_config on an Ubuntu server. I set my ping period to 30 seconds.
After that everything has been working swimmingly!