The xmessage tool lets you display a popup window to users with a text message inside.
First install xmessage
# yum install xmessage
$ DISPLAY=:0 xmessage -nearmouse "Hello"
where options include:
-file filename file to read message from, "-" for stdin
-buttons string comma-separated list of label:exitcode
-default button button to activate if Return is pressed
-print print the button label when selected
-center pop up at center of screen
-nearmouse pop up near the mouse cursor
-timeout secs exit with status 0 after "secs" seconds