Shows window information. Comes with X.
There are several pages for it, but I have this one working:
Xtoolwait notably decreases the startup time of an X session by reducing the load on the X server and Linux.
It starts an X client in the background, waits for a window to be mapped on the root window, and then exits. It can improve performance for users who start a bunch of X clients automatically (for example, xterm, xlock, xconsole, whatever) when the X session starts.
Property displayer for X.
Alternative: Devil's Pie with its debug mode.
more coming later