QtdSync is an easy-to-use rsync server and client for Windows
The small download includes both the QtdSync client (which is fully portable) and server apps, as well as a sync monitor which you can place in your system tray to keep an eye on things. QtdSync is packed with useful features like full synchronization and differential backups, multi-user support with logins, exclude filters, scheduled jobs, command line support, and e-mail notifications.
Want to sync an external hard drive? No problem -- QtdSync can be configured to back it up whenever you plug the drive in. One word of warning: the installer is in German, but it's easy enough to click through even if you can't read the buttons.
QtdSync is currently in beta, but it performs very well and has been stable on both my Windows 7 x86 and x64 systems. It's totally free, and both the application and open source code is available at SourceForge.