boiling water
Harold Mcgee writes:

Once a liquid starts to boil and is turning to steam throughout the pot — the bubbles of a boil are bubbles of water vapor — nearly all the energy from the burner is going into steam production. The temperature of the water itself remains steady at the boiling point, no matter how high the flame is underneath it. So turn the burner down. A gentle boil is just as hot as a furious one.

he is careful to say "the water itself remains steady at the boiling point" rather than saying "the water itself remains steady at 100 degrees Celsius", since the boiling point varies with pressure.

P (psia) - T (C)
------------------
10 - 89.55
14.7 - 100.0
20 - 108.9
25 - 115.6
30 - 121.3

this means that the temperature of boiling water can depend on the atmospheric pressure as well as whether or not the pot is covered.

what is the effect of covering the pot?

it depends on the diameter of the pot and the weight of the lid. here are some boiling points of water boiled with the lid on, for a given pot diameter and lid mass.

d (in) / m (g) / T (C)
----------------------
6 / 100/ 100.02
6 / 1000 / 100.15
6 / 10000 / 101.45
10 / 100 / 100.01
10 / 1000 /100.05


not much difference for a regular sized pot. even if you put a 10 kg weight on top of your 6" diameter pot of boiling water, the temperature would only go up to 101.45.

but if you somehow shrink the size of the face from which steam may issue, then you can have a serious effect on the temperature. a pressure cooker has a tight fitting sealed lid, and a very small hole at the top. by putting a small weight over this hole, it achieves a pressure of about 30 psia, increasing the boiling temperature to 121C. if you were to double the mass of the weight on top of the pressure cooker, you would get a water temperature of 134.5C. but that is very dangerous to do, because the pot could crack and release high pressure steam everywhere.
- posted by Jesse @ 4:42 PM