SyncToy  is a free PowerToy for Microsoft Windows XP and Vista that provides an easy to use, highly customizable program that helps users to do the heavy lifting involved with the copying, moving, and synchronization of different directories. Most common operations can be performed with just a few clicks of the mouse, and additional customization is available without adding complexity. SyncToy can manage multiple sets of directories at the same time; it can combine files from two folders in one case, and mimic renames and deletes in another. Unlike other applications, SyncToy keeps track of renames to files and will make sure those changes get carried over to the synchronized folder.
Updated: for SyncToy V1.4 for Windows XP and Vista (based on .NET Framework 2.0)