The sun. We literally can't live without it. If it went away, Earth would become cold, and plants could not grow. We'd end up like Pluto with everything frozen solid for all eternity. At the same time, the sun is not entirely benign. We depend on it utterly, which means we also have to put up with it when it throws a tantrum. Occasionally the sun creates all kinds of mischief, and we are forced to endure. For a fantastic reference for solar activity, please see this paper by Dr. Leif Svalgaard.
The sun is up to something. Normally it is just a bunch of hydrogen slowly converting into helium, patiently waiting out the eons. However, all that plasma that makes up the sun produces wild and powerful magnetic fields, which can cause the sun to behave erratically.
The sun is currently doing two things it shouldn't: The first is that the overall sun is going 'quiet', with fewer sunspots and less energy radiating. It has been well known that the sun is becoming less active and the Earth is recieving less radiation, especially in the UV range. Solar cycle 24 (the one we are currently in) has been a solar maxiumum, but a particuarly weak one. Solar cycle 25 is predicted by some to be extremely weak, perhaps even a 'grand minimum'. A solar grand minimum was the cause of the little ice age from the 14th to ninteenth centuries. Among other things, the little ice age wiped out the Viking settlements in Greenland as the ice returned. We do not currently know if we will have a solar minimum, or if it will be as bad as this, but due to the inherently catastrophic nature of such an event it warrants closer study.
The other thing the sun is up to is flares. Now, we are heading towards solar maxium, sometime around the end of this year, so more flares are expected anyway. However, flare activity has been unusually strenuous, leading some to believe that there is reason to be afraid. Even fairly minor flares can interfere with communications; a very large one might even destroy civilization itself. A flare like the most powerful one recorded, the 1859 Carrington Event (see post below), could destroy all the electrical infrastructure across an entire continent. If hundreds of millions of people are without electricity for years, then the ensuing riots and famine would quickly overwhelm the globe, and civilization would likely be set back a few millenia. Currently, we have no defense against this sort of thing and only incomplete ways of prediciting it. Our best shot is to hope it doesn't happen until we learn how to shield our technology.