What is behind a Rindler horizon? Or is there nothing? Most of my friends say that a Rindler horizon is an artefact of acceleration. Space continues behind it.
A few of my friends say that the Rindler horizon is an event horizon and a particle horizon. Nothing can be said to be behind it, like for any other horizon, because anybody that sees the horizon cannot observe anything behind it.
Who is right?
 A: 
A few of my friends say that the Rindler horizon is an event horizon

Your few friends are incorrect. An event horizon and a Rindler horizon are both Killing horizons, which is a local property. So locally they are indistinguishable. However, globally an event horizon does not "escape" meaning that it does not connect to future null infinity. Since the Rindler horizon does escape it is not an event horizon.

Nothing can be said to be behind it, like for any other horizon, because anybody that sees the horizon cannot observe anything behind it.

This claim is equivalent to saying that nothing can be said to be in the future because nobody can observe anything in the future. In flat spacetime many null surfaces are Killing horizons, so you are always "behind" all Killing horizons that intersect with your past light cone and outside all the rest.
People making such claims are continually surprised as they go to the future, crossing multiple Killing horizons, and finding that there is indeed more than nothing where they previously thought there was nothing.
A: There can be stuff behind event horizons. After all, most of the observable universe is behind an event horizon due to the accelerating expansion.
You can make the Rindler horizon appear and disappear by changing acceleration. The only thing that changes is what particles and light can catch up with you. It is very much a personal horizon like the terrestrial horizon.
A: 
The Rindler Horizon is the depicted by purple lines (Lightlike worldline [$ x = ct $] not [$x=-ct$]). This centre $O$ is the point when seen from Rindler co-ordinates would appear to be at constant displace if the person had Worldlines indicated in red( Lines of constant ($x'$)).
So at Rindler Horizon, the light pulse seem to be frozen and any observer moving with a proper acceleration will not see anything cross the horizon due to infinite time dilation.
It is a bit different from Event horizon. Due to the fact that we can't see beyond the Horizon, from our perspective we cannot tell whether there is anything behind it or not. But from the inertial frame as indicated in the diagram we can tell there are events that are possible beyond the Rindler Horizon.
