SyncToy is apparently a new power toy for Windows XP to make synchronising files between computers easier. It's not available for download yet but will be freely available from microsoft.com once released.

For now, there is a whitepaper that explains what SyncToy is and what it does for you:

SyncToy is a free PowerToy for Microsoft Windows XP that provides is 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.

Update: SyncToy is now available to download.