imagine.ffi is a foreign function interface based on libffi. It's a binding between Lua 5.1+ and libffi, allowing the user to interact with any other system from Lua. You can use the imagine.ffi module to call DLL's, but also to allow other API's to call your own code through trampoline functions.
imagine.memory is a memory management library, allowing you to manage and interact with memory addresses. You can allocate, read from and write to memory. This module also allows you to specify structures with default C alignment, create arrays of these structs and manipulate complex memory structures.
The imagine.zip module (ImagineZip for AMS) provides you with functions for working with zip-files. This library can be used to extract or create zip files on disk and in memory. You could work with an archive and send it off without it ever touching the disk. Even adding files can be done completely in-memory, but also from files that exist on disk.