I've got a couple folders of personal scripts and such I'd rather not lose if the hard drive on my dev machine goes out (all the source is in SVN, and all the other important stuff is backed up elsewhere as well). What's the easiest way to back up certain folders to another local HD at something like 2AM every morning?
I'd use SyncBack by 2BrightSparks. I've just checked out the freeware version I've got installed on my machine, and it does support scheduling. I use SyncBack for all my backups at the moment, and it works great. The pay-for versions give you many more options including using full regular expressions to specify files/folders, but the freeware version has been adequate for me.
xcopy and Scheduled task
If you need something more user friendly or more features, you might like to take a look at MS SyncToy
Robocopy and and the Windows Task Scheduler. MS doesn't seem to offer a standalone copy of robocopy.exe. You can get it as part of the Windows Server 2003 Resource Kit Tools.
I recently learned that it is one of the standard command line tools in Windows Vista and Server 2008.
Use Mozy. Its a freebie upload-to-the-internet thing that works really well. You can encrypt the files using your own key too, which is nice.
Always backup your precious files off-site, to another local harddrive is not going to help you any if you come home to find you've been visited by burglers.
If you're worried about HDD failure, get Acronis of Ghost and backup the entire drive - replacing Windows isn't something I enjoy doing.