Focus follows mouse

For the last 15 years or so, I've used window systems set to Focus Follows Mouse starting X11 at University, then with an extension to OS/2, then with FVWM under Linux and then Gnome under Linux. By default Mac OS X (and Microsoft Windows) uses Click to Focus, which if you work with a lot of text windows (or applications that involve lots of windows) involves a lot of clicking into windows to focus them in order to do some small thing, followed by repeating that to switch back to another window. (Using, eg, The GIMP in a Click to Focus environment, especially without click through (ie, the first click focuses, a second click is needed to activate something in the window), involves lots of double clicking.)

I'd put up with OS X lacking Focus Follows Mouse for a while with increasing frustration (especially when more often than not the click to focus was being interpreted as a right click, bringing up a menu and not focusing the window). But it turns out in the two instances that it matters most, OS X can be set to Focus Follows Mouse.

In X11.app, Preferences -> Windows -> Focus Follows Mouse can be used to enable the desired behaviour (only on OS X 10.6; OS X 10.5 doesn't have a "Windows" panel in the Preferences pane, but appears to have a hidden preference). That same pane also allows configuring the click to go through to the inactive window (avoiding the double click if not using Focus Follows Mouse). Those settings make using X11 applications like The GIMP much more pleasant (ie, similar to using them on native X11 on Linux).
In Terminal.app there is a hidden preference which can be used to set to allow focus follows mouse. The magic is:
```
defaults write com.apple.terminal FocusFollowsMouse -string YES
```
and then restarting Terminal.app (apparently works in 10.4, 10.5 and 10.6; although some people found issues on 10.6 with being able to Cmd-Tab into other windows, which seems to relate to interaction with Spaces; it appears they found Focus Follows Mouse pretty aggressive about stealing focus if the mouse covers a Terminal window, even if Terminal is not the foreground application -- but in my testing on 10.5 and 10.6 the Focus Follows Mouse only picks amongst the open Terminal windows when Terminal.app is the foreground application, which is probably the most compatible way for it to work).

(There are also some more general Focus Follows Mouse solutions, including MondoMouse (commerical app). And a minimal extension (SVN repository only) suggested in a comment on Val Henson's LJ. Also CodeTek Virtual Desktop Pro, which some comments suggests hasn't seen much maintenance recently, and doesn't work with anything newer than 10.3/10.4.)