This is a pretty diverse genre so I will limit myself to those I have actually played.
So here it goes:
Heretic (1994, PC, MAC)
This was basically a fantasy "Doom clone" made by Raven Software and used the same engine as Doom and was mostly focused on projectile attacks. (i.e. shooting)
The perspective was 1st person like in Doom but one could also look up and down. (quite a feat at the time :P )
The game had a good difficulty curve and many, well-designed levels and diverse enemies and powerful "guns".
It is considered a classic of it's (today not very common) genre and may be either enjoyed in it's original DOS form (I recommend DosBox to run it) or with various enhancements.
Hexen (1995, PC, MAC, PSX, Saturn, N64)
The next title in Raven's line of fantasy shooters/hack n slash was Hexen: Beyond Heretic.
The game used an updated Doom engine and allowed for three selectable characters with different capabilities - fighter, cleric and mage.
It also introduced melee attacks and was ported over to console systems.
The level design, enemy variation, difficulty and replay value were very good and it is considered a classic.
There also exists a mod for Doom 3 which re-creates Hexen in the Doom 3 engine and you can check it out here.
Witchaven (1995, PC)
A similar game to Heretic and Hexen that wasn't made by Raven was Witchaven. (made by Capstone)
The game focused more on close-range combat and it was pretty enjoyable.
The graphics looked very good for the time too. (Duke 3D engine aka Build Engine)
In addition, there was also a multi-player component to the game.
The game got a sequel titled Witchaven II: Blood Vengeance in 1996 which was also pretty good but not much different to the original.
Hexen II (1997, PC, MAC)
Hexen II was the continuation of the first Hexen, this time running on a (modified) Quake engine and featuing full 3D, accelerated visuals.
It's pretty similar to the previous game but features numerous improvements like a bit less linear levels, destructible environments, some RPG elements, ect.
While today it may look a little dated and formulaic, it was an enjoyable game in it's time and can still be enjoyed by the fans of such games or those interested in them.
Heretic II (1998, PC, MAC, Amiga)
The sequel to Heretic took a different turn in that it became a 3rd person action-adventure.
It was using a modified Quake II engine and was an enjoyable game but it was not warmly received by the fans of the original game and underperformed in sales. (despite some good critical acceptance)
This is proably also the reason why there was never a third Heretic game.
Rune (2000, PC, PS2)
The next game which I enjoyed was the Nordic-mythology based Rune.
The game featured a young viking character named "Ragnar" who had to fight thru legions of enemies and monsters.
It was running on the Unreal Engine and had some pretty nice lighting and textures.
The story is quite interesting but the "meat" of the game is the (brutal) fighting system which gives the player weapons like axes, swords, hammers, ect.
The fighting mechanic was simplistic yet effective and the battles enjoyable.
The game also had a multi-player based expansion called "Halls of Valhalla" and was ported over to the PS2. (whose version is subtitled "Viking Warlord")
Sadly, there was never a sequel to the game. (altho there were plans for it)
Severance (2001, PC)
The PC-exclusive Severance: Blade of Darkness was pretty much what Dark Souls is today.
A tough, gritty and brutal hack n slash action adventure game with some RPG touch to it.
The fighting mechanic and control may seem a little clunky at first, but once used to it, it works fine and shows it's deepth.
The game takes place over several different locations and features four different characters with different abilities.
Unfortunatelly, the game flopped in sales despite it's qualities, never resulting in a sequel or remake but got a "cult following" among fans nonetheless.
If you're into such games, it's more than worth checking out.
That is about it from me on the subject.
Did you play any of those or similar games and how did you find them?
If you want to show us some similar "hidden gem" or game that you enjoyed - go ahead. ;)