Suppose that you've got a car with an exhaust gas heat recovery system, such as Honda's or BMW's steam hybrid designs.
Make the steam condenser powerful enough that the water coming out of it is below 100C, and make the steam generator efficient enough that the exhaust gas that passes through it also
cools to below 100C.
The result of this will be to cause the steam that was created by the combustion of gasoline to become liquid.
Pass the exhaust stream through a gas/liquid seperator, and this water can be collected and utilized.
What to use it for? Water injection!
In an otherwise unmodified engine, it has two main effects: it lowers the peak temperature, reducing NOx emmisions, and it has an antiknock effect, either allowing use of a lower octane (and thus cheaper) gasoline, or allowing higher levels of turboboosting/supercharging.
If you don't mind designing an engine that cannot run without water injection, then you could increase the compression ratio to a level that would normally (without water) result in excessive knocking. The increased compression ratio of course results in higher efficiency and power.
Incedentally, most of the water injected should presumably be recoverable in the exhaust -- there's no need to limit ourselves to the water created as a combustion byproduct.
Now that I think about it, the only reason the water created from combustion is important, is because without it, we wouldn't be able to recover 100% of the injected water.
If buring hydrocarbons didn't produce steam, we could still used water injection, and perform water recovery, but some H2O would inevitably be lost in the form of humidity on each cycle, and the water would eventually run down and need to be replenished. But the combined condensate from water injection steam and combustion created steam (even after loss in the form of humidity) should be able to equal or exceed the mass of water that was injected.