Hotkeynet, Keyclone and InnerSpace will all do this for you. I used hotkeynet for 8 months (loved it) before switching to InnerSpace partly b/c AOE ground targeting is so good.

The main different with hotkeynet/keyclone and InnerSpace+ISBoxer, is that with the HKN/KeyClone you get mouse click broadcasting (only the click is broadcast) and with ISboxer I now get mouse broadcasting (the mouse is broadcast in each window - when i move my mouse down in the main, i see the ground target moving down in all the other windows). I get a lot more accuracy. Not sure if HKN or KeyClone can do true mouse broadcasting ... may not be an issue for you if you don't need to worry about precise ground targetting though ... I run a pally,priest,3xmage so it works great for me.