The title says A website devoted to the principles and practices of data compression. Can’t ask for anything better than that. Appears to be run by Nam Phamdo from SUNY Stony Brook. Links to papers, explanations, and software.

User comment:Gives technical explanations and animated examples of a small number of compression methods. The explanations are good, but the amount of coverage of the field overall is small.

http://www.data-compression.com/