yaf-splash pops up a window of text, auto-sized and centered on the screen, that is dismissed when you click on it, rather like xmessage. Unlike xmessage, it lacks buttons and other window manager cruft, can appear transparent via the SHAPE extension, or overlays if your xserver supports them, and has a bunch of other configuration options to tart it up with.
It has a GNU Savannah page, but if you're here, you probably want to cut straight to downloading the latest stable version.
Here are some more screenshots of yaf-splash making itself useful: