It seems that certain nodes, as part of their function, will clamp their output to between 0 and 1 regardless of the input number. I am not entirely sure of this. Is there a list of what nodes will do this in Cycles and the Compositor?
Most nodes should not.
Cycles works under a scene referred rendering model, which means that there is no notion of white or black, but merely a range of values that traverse from some low value to a theoretically (within the quantization limits of 32 bit float) infinite range of light.
As such, all nodes should never clamp, or they will not deal with the scene referred data correctly.
Some by design might map such as the Range node. The Colour Ramp sadly is based on a fixed 0.0-1.0 range which greatly diminishes it's usefulness with modern renderers.
The precise answer to the best of my knowledge, is that there is no comprehensive listing. This is something anyone could map out however via the wiki with a table and a little help from contributions.
Some nodes clamp input values and some intermediate calculations to sane values - often between 0...1 or -1...1
The only nodes that clamp output values should have a clamp option that the user needs to enable for the output to be clamped. I think this list is limited to Math and MixRGB in both material and compositor and the Map Range and Map Value available in the compositor.