Is there an addon that lets me alt-left click a target in the environment ..

[tobindax]

Is there an addon that lets me alt-left click a target in the environment and perform an action?

e.g. "Locate a player of the opposing faction visually, alt-left click on him while hovering, cast <spell name> automatically".

Notice this is not about hovering with any key, but left mouse key.

[Vis]

Clique would be your best bet I think.

http://www.wowinterface.com/download...08-Clique.html

[Haleth]

This can be done with macros.

/cast [@mouseover] [mod:alt] yourspellname

Edit: My bad for not reading properly. As for clicking, I'm not sure that's possible because casting functions are protected. You can do something like bringing up a menu though.

[lordkarthas]

You can use BindPad to bind a macro to Alt-Left click.

I do something similar to this on my tank:
/cast [@mouseover,harm,nodead] Taunt