From Hydrogenaudio Knowledgebase
OptimFROG is a free lossless audio codec, known by its very high compression ratios. OptimFROG is developed by Florin Ghido.
OptimFROG usually obtains the best lossless audio compression ratios on comparisons against other lossless encoders (practically a tie with the current champion LA). 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 its downsides is that there is no source code available, nor format specifications. Another downside is that it is relatively 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.