WinRAR is a shareware file archiver and data compression utility by Eugene Roshal. It is one of the few applications that is able to create RAR archives natively, as the encoding method is held to be proprietary.
In the Frets On Fire community, WinRAR is typically used to decompress/unpack downloaded songs or mods. Conversely, it is also used to compress/pack songs or mods for sharing.
*Note: WinRAR is technically not freeware, but if the trial period ends the program continues to work. It will have a screen that pops up when you enter WinRAR that asks you to buy it. This can basically be ignored.