UnRarX is a simple freeware utility for expanding .rar archive files and restoring corrupted archives with .par and .par2 files. The Mac OS no longer includes software for unpacking .rar files, so if you want to expand these kinds of files (usually from PC users), you'll need to download some sort of unarchiving app to do so.
UnRarX has a fairly crude interface--a single log window with options to test, browse, extract, or enter a password for protected archives--but its drag-and-drop simplicity and high success rate have made it a popular choice for users who want a reliable, bare-bones solution. Thanks to format irregularities, not all unarchiving software is created equal, and UnRarX is one of the more robust options available, especially among the free options. Novice users may have some initial difficulty with the basic interface and occasional quirks (for example, if you want to turn off the "Overwrite existing files" preference, you'll have to reset it each time you open the app). Fortunately, UnRarX includes limited but adequate help.