I use robocopy from Microsoft's server administrators resource kit. I'm still thinking I'm using the best tool after having read this article.

I'm using Allway Sync which is a freebie, and USB Detect & Launch to automate the sync process when my backup HD is attached.

A few years back I beta tested the windows GUI for Synchronex by Xellsoft. Based on Python scripting for the power hungry or point and click simplicity for the timid are both available. Dozens of backup and Synchronization options are at your beck and call just by using the built in wizards - including Webdav, ftp, sftp, lan, internet, xvs, shadow copying, zip and the list could go on! It includes integration with the windows scheduler and to top it all off there is a Linux version as well!

I have used Synchronex since the close of the beta and I will continue to use it to maintain a secure sync across my own LAN as well as the internet for the foreseeable future.