It’s harder and harder to come up with these good computer utilities, but here’s one I use just about every day: WinRAR. It’s a file compression/decompression utility that supports many file formats, with a nice GUI. However, it’s main use for me is its command-line options: it has a very powerful set of options, and it is what I used to back up my data files on a nightly basis. WinRAR isn’t free; it is shareware, and although it has a nag screen it doesn’t keep you from using it. It’s so vital for me, though, that I gladly paid for it.
If you want a totally free version, 7-Zip fits the bill. It’s almost as full-featured as WinRAR, and for a while I tried it out. The command-line options are not as good, however, which is why I stopped using it. If you are just looking for a Windows graphical file compressor/decompressor, however, 7-Zip is perfect.