OptimFROG is a free lossless audio codec, know by its very high compression ratios. OptimFROG was developed by Florin Ghido.
OptimFROG usually obtains the best lossless audio compression ratios on comparisions against other lossless encoders. It has Windows and Linux versions, fully featured input plug-ins for the foobar2000, Winamp2, Winamp3, dBpowerAMP, XMPlay and XMMS audio players (with bitstream error resilience, ID3v1.1 and APEv2 read tagging support), optimal support for all integer PCM WAV formats up to 32 bits and an extensible streamable (error tolerant) compressed format.
One of it's downsides is that there is no source code available, nor format specifications. Another downside is that it is very slow at both encoding and decoding.
OptimFROG uses a new audio compression technology, the generalized stereo decorrelation concept, which was first introduced with OptimFROG 4.0b in December 2001. At the time of its introduction, the new technology yielded significant better (~1.5%) compression than existing state of the art lossless audio compressors.