By changing your Xsession file you can start a kiosk-style machine that starts a single X11 application.

On Debian you would change the file /etc/X11/Xsession.d/99x11-common_start. In stock form there is a line that reads:

   exec $STARTUP

Change that to read:

   exec firefox javascript:%20resizeTo\(1024,768\)

and your machine will boot directly into Firefox at a screen size of 1024x768. Of course you can change the size to any resolution you like. You can probably also add more javascript as well and force opening a web page and other stuff. Combine this with Bastille and an autologin of a non-privileged user and you'll have a very convenient (and cheap) kiosk.

On some Linux distros the Xsession file may actually be a single file rather than a directory of files. A search such as:

    find /etc/X11 -iname 'Xsession*'

should find it for you. The actual line to change may differ also but should be near the end and be an exec statement or a function call that invokes exec.