PECompact (if you don't know) is a compressor for executables. I think it is probably the most versatile compressor, with a modular format that makes extending it's functions simple and effective.
There are simply loads of different compressors, codecs and loaders available for it, every popular compression algo has a codec provided. Also it has built-in support for hook modules, allowing you to extend the Windows Api, add virtual files or do pretty much anything you can think of.
I have written a number of plugins for PECompact since 2005, here you will find a few of the public ones.