RAR Expander is a free utility for expanding archive files with the .rar format (typically received from PC users). The Mac OS used to come bundled with software that could expand .rar files, but now you need to install a separate program--such as RAR Expander--if you need to work with them.
RAR Expander doesn't have a particularly pretty interface, but it expands files quickly and reliably. All you have to do is choose Expand from the app's File menu (which is the only command in the app's File menu), browse to find the .rar archive that you want to expand, and hit return. RAR Expander will prompt you for where you want to expand the file, or you can set the preferences to handle this automatically--and the preferences can also determine whether RAR Expander creates a surrounding folder, checks for updates, or beeps on completion. This app also comes with rudimentary AppleScript support, along with two sample scripts.
There are other free .rar utilities that have slightly more functionality (such as the ability to restore corrupted archives), but RAR Expander can handle most simple expanding tasks.