XELB (X protocol Emacs Lisp Binding) is a pure Elisp implementation of X11 protocol based on the XML description files from XCB project. It features an object-oriented API and permits a certain degree of concurrency. It should enable you to implement some low-level X11 applications. Please refer to xelb.el for more details.
Note to Emacs 24 users:
If you install XELB from source (rather than GNU ELPA), be sure to install
cl-generic package from GNU ELPA first.