If you've had your computer for more than a few weeks, there's a good chance that your hard drive is speckled with redundant data—files and folders that, for one reason or another, live in more than one location on your PC. Worse, sometimes you wind up with two versions of the same document. (Thanks, email chains!)
Enter Winmerge. This handy program lets you compare two folders to sniff out repeats. Even better, the program lets you merge two mostly similar files into a single, comprehensive file, if you're dealing with code or documents (though you may need to download the xdocdiff plugin to coax Winmerge into reading Office files). It's seriously handy.