Our planet, Earth, is as far as we know, the only body in the where life has managed to evolve and survive to the present day. It is probable that the conditions required for non-living matter to arrange itself in such a way that it becomes alive are very unlikely to ever be met at all. The universe is a hostile, dangerous place, and Earth just happens to be relatively benign.
What are the very absolute basic requirements for life to evolve? Life needs three things: an energy source, a liquid solvent, and a source of carbon. Too close to a star and there is too much energy; too far away and there is too little. A tight range of thermal energies must be sustained for liquids to form. Carbon-based organic molecules can be form spontaneously in most places, but unsurprisingly, their synthesis requires the right chemical and energetic conditions. These all need to be present together for long periods (i.e. millions of years).
However, though it is true that it’s extremely difficult to create life, it is not so difficult to sustain life once it has been created. Life is extremely adaptable: microbes can been found living deep at the bottom of the sea, high in the upper levels of the atmosphere, and even inside solid rock. Humans, through use of technology, can also survive in tough conditions (though perhaps not solid rock).
So where else could humans survive? With the right shelter, we need a temperature range of roughly -40°C to +60°C; we also need water, and a place to grow food. A great number of places in the solar system meet these conditions.
The moon has no atmosphere, and so experiences wild swings in temperature, but has some areas that are always warmly lit, and some that are always shaded (and contain ice). Experiments using the soil brought back for the Apollo missions have shown that it is excellent for growing asparagus. The next easiest location, asteroids and comets have often have lots of water and valuable minerals, and landing on them is very safe due to the weak gravity they possess. Lots of tethers will be required, of course.
Mars has a protective atmosphere and water, and is cold, but not too cold. It has a lot of earth-like soil and is relatively easy to reach from Earth. Martian days are the same length and ours, and it experiences seasons just like ours (though last twice as long).
On Mercury, high noon is far too hot (+400°C) for us, and midnight is far too cold (-200°C), but twilight is quite comfortable. Due to the extremely slow rotation of Mercury, the land experiencing twilight moves at 6mph: a vehicle could easily follow the comfortable zone as it moves around the planet. Similarly on Venus’ surface is far too hot, but as the planet has an extremely dense atmosphere, an airship containing Earthly air would float comfortably among the warm Venusian clouds.
Much further out, the Jovian moons are stretched and squeezed like stressballs by Jupiter’s immense gravity – this makes them intensely volcanic and so surprisingly warm despite their distance from the Sun. Titan, the largest of Saturn’s moons, is the only body other than the Earth known to have liquid on its surface (although it is liquid methane, not much good for drinking).
If you’re willing to put up with a long journey, several planets around the star Gliese 581 may have liquid water on their surface, and so are likely to be ideal for colonisation. Just don’t think we can get there in a hurry: it’s twenty lightyears away.