foobar2000 is an advanced freeware audio player for the Windows platform. Some of the basic features include full unicode support, ReplayGain support and native support for several popular audio formats.

Main features:

• Supported audio formats: MP3, MP4, AAC, CD Audio, WMA, Vorbis, FLAC, WavPack, WAV, AIFF, Musepack, Speex, AU, SND... and more with additional components.
• Gapless playback.
• Full unicode support.
• Easily customizable user interface layout.
• Advanced tagging capabilities.
• Support for ripping Audio CDs as well as transcoding all supported audio formats using the Converter component.
• Full ReplayGain support.
• Customizable keyboard shortcuts.
• Open component architecture allowing third-party developers to extend functionality of the player.

More info at Foobar2000.org