As you've seen, thanks to Lua scripting, Devilspie2 is quite easy to work with. If you want a smaller tool for interacting directly with windows, though, then take a look at wmctrl [5]. This is a "command-line tool to interact with an EWMH/NetWM compatible X Window Manager" – which means pretty much all major WMs written (or updated) in the last decade. Examples include Blackbox, IceWM, Englightenment, and Window Maker.

As with Devilspie2, you can use wmctrl to move windows around, resize them, put them on specific workspaces, and do other tasks. It's a more demanding tool to use, in that some familiarity with X Window System underpinnings is important, but the documentation is fairly good. You can even do some more advanced things like changing window titles on the fly and resizing the current desktop.

Another tool that's slightly related is xdotool [6], which lets you "simulate keyboard input and mouse activity, move and resize windows, etc." (i.e., you can create scripts that send virtual key presses and mouse clicks to windows) (Figure  4). It's especially useful for automating certain tasks, such as clicking on a series of buttons in a complicated dialog, so you can even use it for automatic testing of GUI software. Like wmctrl, it's not the easiest program to use out of the box and has a somewhat steep learning curve, but it's worth putting in some time to learn the basics. Still, wmctrl and xdotool are both included in the package repositories of all big-name distros, so you don't have to go far to find them.

Figure 4: Xdotool, a great alternative for automation, sends virtual clicks and key presses to apps.