Dave 2 Huffman compression
Dangerous Dave 2 Huffman compression is the means by which most of the non-tileset graphics are compressed. (Screens and sprites.)
The file is composed of three parts, a brief header, the Huffman binary tree, and the compressed data. The 'header' consists of the string 'HUFF' followed by a dword giving the decompressed file size. The Huffman table is of the usual format, giving a fixed size of 404 bytes before the compressed data.
This format reverse engineered by Napalm
The Huffman Compression page has source code for producing a Huffman table in this format that doesn't decompress data, so the raw data can be copied right after it.