What is the compression format of various skin, themes, game data files?

in most cases there are no compressions programmers normally change the extention from png or bmp to dat
various files including zip or lha mention the compression like zip mentions as pk in the file, lha as lh, rar mentions rar
this is normally stored as text in the first few characters. open the file in any decent editor (if the file size is small notepad will do) and you can identify the compression.

Where can I find samples of these files to view them in a text/hex editor, as suggested?

now thats a difficult question to answer. try hitting download and the extention in google

well, i've had some experience in skins, and so far a lot of skins used in programs like those in Winamp is basically bitmap (bmp), usually you can find freeware that will help you create skins for certain applications. with themes, the same also applies. for games, usually they use their own native format for images, but are usually derived from image file format such as TIFF. game data files depend on the game, but some are actually naming it using "custom" file extensions like MIX, or something. and some can be opened in Notepad much like Windows' config.sys (e.g. some MIX files found on Red Alert games). for examples, look for files with the .mix extension in C & C: Red Alert, some can actually be viewed in a text editor. new games also use mp3 files for sound.

Rod

I think the common used compression algorythm is "Lempel-Ziv". "Huffman" tree compression is out nowadays and RLE is useless for complex structures.
Zip uses Lempel Ziv
Rar too(I'm quite sure) but It does not format the compressed bitstream like zip

other games (quake I, II, III, IV?) have for example .pk? files (this is zip with changed extension). Thief I and II have .crf extension (its zip too).
Operationflashpoint has .pbo files which are packed and encrypted (here you need a special editor)
tga and bmp files are not packed.
jpg is jpeg compressed
and gif are vectors

wav files are not compressed.
mp3 and ogg are mpeg compressed.

did it help?

Great, void_main.  This is what I was looking for.  Let me summarize so far:

1. Quake I,2,3 - .pk?    (Renamed zip)
2. Thief 1,2    - .crf     (Renamed zip)
3. Operation Flashpoint - .pbo    (Propietary packed and encrypted)

I had a hunch that most of the files were renamed common compression formats.  Any ideas which others (including the skin and theme files) are also renamed zip files?   Others renamed .rar, .lzh, .arj, etc.?

katoa : Remember that knowing the format of compression is okay. but if you are looking into changing some data into it take care that you dont cross EULA.