I am searching for a utility that will monitor a given X application's window and alert me of any change. This would be most useful for letting me know that something has happened in a window on another virtual desktop, where the window itself doesn't support such a feature. For example, a meta-refresh happening in a web browser, activity in a tailed log file, etc. (Actually, the Terminator terminal supports activity, so alerting a tailed a log file is done.)

I have a very crude bash script that, given a window ID, takes a screenshot of the window with xwd every second, and issues an xmessage if the screenshot differs from the last one. However, being crude, it doesn't always work.

I've had a look on Freshmeat and didn't find anything that can do this. Does anybody know if there is such a utility out there?

You could try setting the SIGIO server flag option on in the xorg.conf "ServerFlags" section, and trapping it. (See man xorg.conf for details.) Or just modify the programs creating the windows to notify you when the window contents is modified. (Unless you're using a propitiatory program, the source code for the program is available for you to modify to suit your needs.)