Here's a short post about a handy tool. When making targeting macros, you can use a number of modifiers to the macro so that it does different things when you click, right click, alt-click, shift-click, etc. But you can even modify it to do something when pressed while you hover the mouse over something.
For an example, as a Druid Bear Tank I may want to Lacerate my secondary target while staying on my main target when tanking. I could make that secondary target my focus and use a macro that does this:
/cast [target=focus] Lacerate
But what if I want to lacerate something besides my focus? Enter the mouse over macro.
/cast [target=mouseover] Lacerate
Put this macro on an action bar, and assign it a key binding such as J or whatever you want to use. In my example while targeting and tanking the main target, I can hover my mouse over the secondary target (another mob on me) and hit J to lacerate him without switching targets, and then go right back to gaining aggro on the main mob. This is really handy for holding multiple mobs aggro as a bear tank, but it can be modified for a lot of different classes and spells.
Have you used a mouseover macro before? If so, when and where?