Once upon a time... steel frames and no suspension!

Young mountain bikers have grown between suspension forks, aluminium, full-suspended ultralight carbonium frames, but for the previous generation of bikers (the '80 kids) mountain bikes were a simple and sturdy vehicle, with little technology and a 18 or 21 speeds. I suppose that a lot of those bikes are still around, maybe used as bad weather bikes or commuters.
The steel frame was very heavy compared to the nowadays frames, and the reason was very simple: they had to be solid to take a lot of abuse, and this is why a great part of those bikes can still be used. I highly doubt that the modern alu ultralight frames can live so long, due to the properties of the alumium itself (it does not bend), as it seems to break after few years of abuse. I mean, it's not that an alu city-bike will break so easy, but a frame made for off-road will be more inclined to break.
There are still some no-suspension steel framed bikes around, but most of them are crap.
But, if you are in some way nostalgic of the old school bikes, or if you are a young biker and want to time-travel to taste the mtb-world of the late eighties, this beauty is for you:

The Btwin Rockrider 5.0, you can find it in all the Decathlon stores. Well built, sturdy and cheap. Exactly what a mountain bike should be.
If you are a minimalist (or if you live in a flat area), there is also a singlespeed version called Vitamin:

Less things on a bike means less things to break and lower maintainance.
Enjoy!