Matfile
MAT-File is a format used by Mathworks' Matlab for storing formatted data.
This is a proprietary, binary format that appears to be fully supported only by Matlab. However, GNU Octave, an open source alternative to Matlab, should be able to read and write the format as well.
The published specification makes a distinction between Level 4 and Level 5 MAT-files, where Level 4 files are compatible with older MATLAB versions (up to version 4), whereas level files are compatible with MATLAB versions 5 and higher. The overall layout of a Level 4 file is quite different from Level 5, and the two might even be considered separate formats. For example, Level 4 files don't have a unique 'magic' byte pattern that would allow easy identification, whereas the header of a Level 5 file includes a descriptive text field that could be used for this.
