DoubleKiller

The program doesn't just look at filenames; it compares the filename, size and does a quick scan of the files to find the real duplicates. And once you've found them, you can either delete the files or move them to another location, freeing up the space with a couple of mouse clicks.

However, be careful: on a typical system, this program will find hundreds of duplicates, but that doesn't mean that you should delete all of them: some are useful (such as programs that install files into the Windows directory, but keep a copy in their own directory as well). So, only remove the duplicates you are sure that you don't need. This is the free version: There's also a Pro version which does more sophisticated searching, scans faster, but costs $20.

--Richard Baguley