An addition to current engine technology?
 

One interesting idea might be to use the heat-up / cool-down process of engine coolant pro-actively... That is, once it is heated up to divert this 15 or so psi of pressure into a steam-powered turbine which in turn exhausts the coolant back into the system...

The trick here is to not use the water-pumps flow generation to turn the turbine, but just the fact that this hot coolant seeks expansion. I am still not sure how to re-introduce the coolant back into the system without a loss due to possible compression on the other end...

Sort of like an air-conditioning system, but rather than having a compressor we would use the engine to create the heat, which it already does... Perhaps the turbine would thus exhaust directly into the radiator which, since that cools the mixture might have a suction effect.
Granted, the radiator wouldn't quite suck the coolant, but since it does cool the mixture it stands to reason there exists less pressure there, so the water is moving from the HOT (high psi) end to the COLD (low psi) end via a turbine.

All right:
Coolant enters the engine at a certain temperature and gets heated.
This heat creates pressure which pushes the coolant into a steam-like power turbine.
Which then exhausts it back into the radiator that in turn cools it right off.

It wouldn't create a ton of energy, but it might be enough to power an alternator that could now run independent from gas-powered engine propulsion, or it might be enough to self-propel or at the very least take all pressure off the H20 pump, if it does this without counter-acting itself again in terms of weight gain.

There remains the issue of back flow (or the other end of this psi equation since it's an open loop), the water pump can not be relied upon to counter act this effect because doing so would only balance out the gain back onto the pump (which is powered by the engine) thus negating the effect.
And thermostats, we would need something that once the engine reaches a certain temperature always retains some kind of an opening.

Yes, this would be counter-productive on cold engines...
And I have to hope I haven't just created yet another false 'free energy' theory. :p

See BMWs website. They are already putting significant effort into heat recovery systems that have shown a significant improvement in FE.

The problem here is that we are talking about pressure of a liquid. And since the expansion of a liquid when pressure is released is not that great, there is VERY little energy to be gained here. On the other hand, it is (in theory) possible to recover some energy from the coolant by using a Rankine Cycle engine uilizing a fluid like ammonia or a hydrocarbon. BUT, such an engine would weigh quite a bit and take up ALOT of space for the power that you could get from it. It would also be expensive. So it is unlikely that something like this could make it into widespread use.

Just thinking, 90% of the right parts are in the air conditioning system, they should get it figured out how to make it dual mode and provide a couple of HP to the crank... would need something like a rotax vane compressor...