Increase thermal efficiency of combustion engine by using heat of coolant/exhaust? I can't be the only one who's ever thought of this, but obviously it hasn't caught on:
In terms of energy density, fossil fuels are the best thing around short of enriched uranium (and, flammability aside, a lot safer). The biggest two problems with using fossil fuel as an energy source are the low thermal efficiency of combustion, and the production of CO2 gases. The second problem is the more publicized, but it can be mitigated to a very large extent by solving the first.
One idea, which may catch on in industrial power generation, is the "carbon-conversion fuel cell", which at least in the lab gives a thermal efficiency of about 80% of the theoretical energy potential of the fuel. However, the energy density (1kW of generation capacity per square meter of fuel cell surface area) and the operating temperature (750*C) make it impractical at the small scale.
However, we currently have technologies that can improve efficiency of combustion engines, we just aren't using them. ICEs typically top out at about 30% thermal efficiency; the rest is lost, primarily as heat, some as kinetic energy of expanding gas. We even go so far as to build cars with water coolant loops and mufflers to more efficiently waste that heat and kinetic energy. Well, what if we instead harnessed that energy to do more work for us before it leaves the car? Imagine a parallel hybrid with two additional generators in addition to the one powered directly off the drivetrain of the engine. One would be powered by steam from the superheated coolant (anyone who's opened the radiator cap on a hot engine will tell you this system is perfectly capable of producing copious amounts of pressurized steam), and the other would work much like a turbocharger, using the pressurized exhaust to turn a turbine, but instead of an intake compressor the turbine would power another generator.
To my knowledge, no car manufacturer has ever fielded a production passenger vehicle that utilizes these additional energy sources to charge the battery system. But these technologies are decades old; one would think that we have the technology to pass the breakeven point on power to weight with these systems. Does anyone have documentation showing these additional avenues of energy capture have at least been considered, and if so, why they're not being pursued more aggressively?
 A: What you are talking about is called a combined cycle engine.  They are commonplace in stationary power generation, i.e. utility-scale electricity generation.  There has even been some talk of combined cycle engines in cars.
As pointed out in the answer by dmckee, the reason this hasn't been widely applied in cars is that no one has demonstrated an economically competitive combined-cycle car. I promise you, if such a thing can pay for itself in gas savings then it will eventually be built and sold, unless some better technology makes it irrelevant.
In general there are many reasonable ideas that are physically permissible but economically or technically difficult or nonviable. You are effectively suggesting to add a steam engine to a car, which is quite a difficult proposal. I'd suggest that a hybrid gas-electric car is more economical than what you suggest, and even they have had a hard time catching on.  In electric power generation it matters much less that the combined cycle engine has a larger sunk cost than a normal engine, is heavier, etc., so the economic balance works out.
Bringing the question back to physics, no matter what you use for heat scavenging, your engine including all of its "subengines" cannot exceed the Carnot efficiency corresponding to the largest temperature difference in the engine.  Adding additional heat engines will help to approach the Carnot limit.  In order to beat Carnot, you can't use heat as an intermediate step between chemical energy (fuel) and mechanical work.
A: Recall that in a car you have not only the extra capital cost and extra maintenance (which all power system must be concerned about) but must also carry the extra weight of and find the extra volume for the additional system. And this being an engineering question, not a physics one you must then ask "Is this worth it?"
Recall also that the extra weight probably means beefing up the suspension (which adds more weight) or reducing the working load.
Recall that extra volume may mean making the car bigger (and therefore heavier) or less streamlined.
Recall that if you deploy novel systems that have an effect (or in this litigious times) even a perceived effect on safety there will be cost and maybe fewer sales.
Recall that each of those additional systems runs on a much smaller temperature difference than the primary IC engine and therefore has a lower theoretical maximum efficiency.

Our cars have traditionally run on IC because you get a lot of power per unit weight and volume, not because these are maximally efficient systems. In the early days of motoring steam and electric were both tried. They lost the economic battle.
