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A .rm file is a Real Media streaming file. Developed by RealNetworks, it remains one of the most popular streaming formats for audio and video in the world. It's quality however fails to impress when put against newer video and audio formats.
Real Alternative is an excellent player for Real files.
Related software tools
Real Alternative (Freeware)
Real Alternative will allow you to play RealMedia file without having to install RealPlayer or RealOne Player.