WinFF
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An open source media converter. Can convert between most video and audio types. WinFF is the GUI for the underlying FFMpeg utility.

XBoard and WinBoard
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Open source UIs for playing and replaying chess games on. XBoard is the Linux version, WinBoard the Windows version.

GSpot
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Identifies the codecs needed to play media files.

MP3Gain
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An open source mp3 volume normaliser. You can set the volume you want the mp3s to normalise at. and do them in batches.