A good place for a general list is http://www.engineeringtoolbox.com/melting-boiling-temperatures-d_390.html
For high temperatures, here is a list of alloys https://en.wikipedia.org/wiki/Fusible_alloy Some of these have melting points below 0 °C and boiling points as high as you will ever need. Some are pretty expensive, though. Note that in general mixtures have a lower melting point than pure substances. The mixture whose percentage composition has the lowest melting point is called a Eutectic.
As a pure metal, tin is a good choice as it has low toxicity and a wide range. Hence it is used in the float glass process for window glass. Even though it is not particularly reactive, there is some potential for an explosive reaction of molten tin with water/steam if used in large quantities, partly due to the liberation of hydrogen.
The solar energy industry has used sodium and potassium nitrate (and previously sulfuric acid) but both are corrosive. Additionally, nitrates may be difficult to get hold of outside the agricultural industry in some countries as they are used in making explosives.
Then there are the organics. Silicone oil and Dowtherm A have been mentioned. Neither is particularly good for you. Cooking oil doesn’t have a bad range and is preferable to Diesel from both toxicity and flammability points of view. Among the alcohols, ethanol and methanol are probably too volatile for your use. The glycols (diols) are used as antifreeze additives with water. Ethylene glycol is quite toxic. Propylene glycol is much less toxic. Glycerol (a triol) is also nontoxic and higher boiling than propylene glycol, but it is highly viscous which will negatively impact its heat transfer properties.
It should be remembered that non-toxic organics can decompose into toxic ones, particularly when the heat flux applied to them is too high (as can happen with electric heating elements.)
Fun fact: Propylene glycol is much less toxic than ethylene glycol because the latter is metabolized to oxalic acid, whereas the former is metabolized to lactic acid, one of the most common substances in the body.