Put a Utility set to phase invert on the "A Return" track.
Put an effect (100% wet) on "B Return" (try a ping pong delay to get a feel for it)
On the A Return track turn up Send B to maximum and set the gain to 0db
Make sure that both sends are set to post
You can now use send A on any track to xfade between 100% dry and 100% effect. great for dub delays where you want to drop out the original sound but still here the delayed version - for example.
Send B will bring in the effect in the normal way.
You can set up several effects in this way to create a kind of dub pallette, and spontaneously apply any combination of effects to any track (ok, this is obvious

Arguably the 180 degree phase shift might affect the sound of the dry/wet mix with some effects, but another phase inverter placed before the effect would fix it. In practice it doesn't appear to make much of a difference with most effects.
I originally posted this idea on another thread (http://www.ableton.com/forum/viewtopic.php?t=27362) under general, but thought it was worth posting here as well.
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