I don’t do as much downloading from multiple sources as I used to in the heyday of P2P sharing, but I still nab the odd compressed/archived file from time to time. While OSX does have the ability to natively open ZIP files, there are plenty of other formats you may encounter that your Mac can’t do without help from an app.
Regardless of what kind of archived file you need to open, The Unarchiver will do the job. The small (3.9 megabytes) app quickly and efficiently detects and extracts your data from just about any format of compression, from ARC to ZIP. The list of compatible filetypes is as impressive as it is comprehensive. Want to peruse the contents of an archive without unpacking it? No worries, The Unarchiver has a sibling program to do just that: the Archive Browser.