xosd is a library that implements an on-screen display for X11. Basically, it just takes text, or simple graphics, and shows it on top of other graphics on screen.
It comes with "osd_cat" tool that workd like cat(1) - it shows specified text file on screen on specific location using specific font and color. There's also an XMMS plugin to show player information on specified location of screen.
Some other programs (like "hotkeys", a program for dealing with "multimedia keyboards") also use the library.
Home page: http://www.ignavus.net/software.html - also comes with many Linux dists, including Debian.