Firstly, Rayleigh scattering, which effectively redirects a small portion of light as it travels through a perfectly clean atmosphere. In a pure nitrogen atmosphere at STP, the light you get out of a regular green laser (wavelength 532nm) will have a Rayleigh scattering cross section $\sigma$ of about 5.1x10-31m2. You might expect to find about 2x1025 nitrogen molecules per cubic metre.
Given an initial beam intensity of $\Phi_0$ that travels through $z$ metres of this nitrogen atmosphere with particle density $n$, the intesity of the beam as it leaves $\Phi$ is defined as $\Phi = \Phi_0e^{-n\sigma z}$
This means that a beam of that passes through 1m of that nitrogen atmosphere, 99.99898% will carry on in the direction it was travelling and the rest will be scattered in a random direction. Once that beam of light has passed through approximately 450km, only 1% of the light will have been unscattered.
This has two important effects. Firstly, things that are a very long way away will appear fainter, as some of the light travelling from them will be scattered away and will never reach your eyes (or whatever you are doing your looking with). The second is that the Rayleigh scattering cross section is inversely proportional to the fourth power of the light's wavelength: shorter wavelength (eg. bluer) light is scattered more strongly than longer wavelength (eg. redder) light. This means that things will be fainter and redder. A similar effect occurs in astronomy, whereby distant objects behind clouds of gas and dust appear redder, even in the absense of any redshift effects, because the bluer parts of their spectrum are scattered away.
Computing the exact effects here is awkward, at least in part because the definitions I've found for computing Rayleigh scattering cross sections totally disagree with actual measurements of cross sections, so whatever I'm doing I'm clearly doing it wrong. Nonetheless, it seems clear that even in extremely clean air lines of sight of over 100km aren't going to show you much. Here's an example from the real world, with a 90 mile view.
When the stuff in the way of the light is large compared to the wavelength of that light, you can't use the Rayleigh approximation any more and you need to think about Mie scattering, which is a bit more challenging. Cloud, fog, mist, smoke, soot, ash, dust... all these things cause Mie scattering. Mie scattering from large particles is mostly wavelength independent (which is why clouds are whiteish or greyish) though shorter wavelength light tends to get scattered more.
Actually working out the contribution of Mie scattering is a) really hard and b) requires making a lot of assumptions about the nature of your atmosphere ("Earthlike" has a vast amount of variation), so I'm simply not going to do it.
Instead, I'll steal a table from chapter 15 of Vision and Acquisition (available in its entirety for free here).
Mie scattering tends to be more biased towards scattering in the direction the light was originally travelling, so it won't make things dark so much as make them blurry and faded... effectively, distant objects just disappear into a featureless "haze". You can see that the effects of this sort of scattering can be profound. Precisely quantifying it is very hard, but it is worth noting that with the best eyes in the world you can't "see through" this haze... the light travelling towards you from a distant object has been bounced around and blurred enough that the details are no longer there to resolve. HDE 226868's answer points out that the resolving power of the human eye is limited but artificial aids can help... this should show that they'll only take you so far, and the best visible-light telescope in the world isn't going to help you. YOu need to use longer-wavelength things, like infrared or radar to help you here.
I'd like to say more on atmospheric distortion (for instance, I think you might see a slight bending effect that makes it look like you're in a very broad, shallow dish instead of on a flat plane) and thermal distortion is also going to ruin seeing things at a distance, but everything is just entirely too hard. Sorry about that!