Windows only: You've waited hours for the download to come through. You jump into your downloads folder, right-click to extract the file, and... ERROR. DiskInternals' free ZIP Repair app mends those files that didn't come through to save you the re-download.
After installing ZIP Repair, you'll find its use pretty simple. Start the app, point it to your broken ZIP package, and tell it where you want the fixed-up file to go. ZIP Repair does its thing, drops off the files, and reports in a list which files were fine, and which needed a little work.
Looking through the archives, we've not really posted many (any?) archive repair tools, so if you know of one that works on formats beyond ZIP, we'll gladly take a link in the comments. ZIP Repair is a free download for Windows systems only.