Since I have a big terabyte NAS drive hooked up to the network and it's important to keep backups of things, I decided to mirror our most important documents from the engineering machines. And there are loads of free folder-mirroring solutions out there. But it turns out that there's one built into Vista (and is available pre-Vista if you have the "Windows Resource Kit" or you just dig around a bit).
It's a Microsoft command-line gizmo called "robocopy", and it's basically xcopy on steroids. It's got millions of options, but the most important thing it does over xcopy is that it'll only copy changed files like a standard disk-mirroring utility. That means that if I have 50 gig of mirrored folders but only three files have changed, it'll only copy over those three files.
So a little batch-writing and work with the standard Windows task-scheduler, and I have a full mirror of our documents. And it's been working just fine for a year now, so I'm happy about it. Actually I mirror the files to a different folder every night (civilgrrl-1/monday/documents, civilgrrl-1/tuesday/documents, etc) so I have a seven-day backup of all of our files.
One good hint. If you're copying files from NTFS to Linux (a distinct possibility as most NAS boxes are running Linux) then use the /FFS command-line switch. When I first tested the robocopy-backup, it was backing up a lot of files that hadn't changed between sessions. Turned out that NTFS's file timestamps were more finely-grained than whatever the NAS is using, and the timestamps weren't exactly the same (just really close). The /FFS switch assumes that timestamps up to 2 seconds apart are the same, which fixed the problem.
1) It runs on Windows, Unix, Linux so I can have the same backup command for all machines
2) You don't have to have the remote drive mounted locally over the network
3) You can easily tunnel it using ssh so you can have a disk at a remote site (in my case friend's house) so you can easily make offsite backups the same way as you do onsite backups.