Roshal ARchive (RAR) is a proprietary file format used by the compression software WinRAR. They make the decompression code available for use in other programs and allow its distribution, but with a license provision that "You cannot use the unrar source to re-create the RAR compression algorithm, which is proprietary."
Current versions start their files with (hex)
52 61 72 21 1A 07 00. Early versions (for which no format documentation is known) start with
52 45 7E 5E.