Is Earth an organism?
[I am going to preface this by saying that I am not in one camp or the other in this discussion, as I think there are pros and cons to each side]

This links to the "Gaia Theory" proposed by James Lovelock, wherein the Earth is considered a self regulating organism. 

In this hypothesis, Earth is an organism in that it will try to maintain certain conditions.  These conditions include temperature (within a range suitable for life as we currently understand it), oxygen content of the atmosphere, water availability etc.

This does not mean that the ball of rock we are flying through space on is a living organism as we understand it, but rather that the various spheres of life (These are usually the biosphere (ecosystems), the atmosphere, the cryosphere (the poles, and other ice-bound areas), the hydrosphere(oceans), and the pedosphere(soil)) interact with each other in such a way that optimal conditions are maintained.

Whilst the theory as a whole is not used today, it is recognised that feedbacks and interactions between different spheres occur, and are an important part of understanding Earth systems, and is studied in an area of science known as Earth System Science

One of the initial arguments for this theory was that our atmosphere is not in chemical equilibrium, and that there must therefore be processes active to maintain oxygen levels at a steady state outside of equilibrium.

This is a difficult topic, as it is important to stress that the Gaia hypothesis does not state the Earth is some sentient being consciously doing anything, rather that a complex set of interactions, abiotic and biotic, combine to maintain and regulate the planets temperature and atmospheric composition.

An example of this regulation is chemical weathering, whereby carbon dioxide in the atmosphere falls in rain, and reacts with rocks, to form calcium bicarbonate, removing carbon from the short term carbon cycles.  The more surface rocks there are which are capable of this reaction, the more CO2 is removed from the atmosphere.
There is a hypothesis which says that at least once, the Earth has been covered largely in ice and snow, and this reaction (known as carbonation) is suspected to have played a part in the recovery of Earth.
The hypothesis (Snowball Earth) goes like this:

The Earth became cooler, whether due to Milankovich cycles, volcanic activity, or solar output changes.  As this happened, ice began to form at the poles, changing the albedo, and reflecting more radiation back to space, this caused local cooling, and allowed for the growth of the ice sheets. As they spread, the albedo became higher on a larger scale, and the ice sheets eventually covered the two hemispheres, with a band of open ocean at the equator.
The earth was now very cold, but not completely empty of life, or without volcanic activity.  Life continued to respire CO2, and volcanoes continued releasing CO2.  Without rocks to remove it from rain, or ocean to absorb it, Carbon dioxide began to build up in the atmosphere.
As it built up (over millions of years), it began to warm the planet, which in turn began to melt the snow. As the snow and ice began to retreat to the poles, rocks, and open ocean were again available to remove the carbon dioxide from the atmosphere, and more photosynthetic organisms were able to survive, and remove more CO2 through photosynthesis.

This lead to a stabilisation of the planets systems again, although, it may have gone too far, and swung back to a glacial period (although not a frozen earth), then moving back to a warm period before stabilising due to the tectonic movement of the continents northwards The break-up of Rodinia, birth of Gondwana, true polar 
wander and the snowball Earth

I hope this long ramble illustrates how there are a number of complex systems which all interact with each other to maintain conditions.  Whether we consider the sum of these systems an organism is an ongoing discussion, but, all organisms have the ability to regulate via various systems, and each of these systems has the potential to lead to chaotic positive feedbacks, and spiral out of control, although negative feedbacks usually keep everything running as normal. 

I have linked the Gaia theory webpage, and if you search for Daisyworld model, you will find a model which you can run to simulate feedbacks
It is up to you whether you think this collection of feedback systems is an organism, and this debate will likely run for decades more.

Gaia Theory Site