I've got few ideas.. However I do not know much about how the damage range and stuff like that works but eh;
The Crowbar
Pick up; "Someone dropped this in the wrong place.."
Damage wise similiar to chainsaw, maybe slightly less but more accuracy to make up for it.
However, I had an idea that this weapon would allow you to break down doors and some walls to give meeleers some ability to break stuff.
Origin; Half life ofcourse
Spur
Pick up; "Ready and perfected"
Your generic little pistol unique. Normal shots do a rather avarage damage, however, this one has a nice little special reload..
This gun has 3 special reloads, each that go by stages. The first one turns the shot into an accurate ray gun like shot, it however takes a while to charge up. Once its been charged up you can charge it up again, which doubles the effect. Afterwards you can charge it up a 3rd time that makes the shot even more powerfull, bit like a massdriver.. It however takes a big load of time to do so.
Also each special reload requires big load of extra ammonation, possibly even power cells.
This would ofcourse, be extreemly rare weapon due of the railgun like nature.
Origin; Cave story.
The UnMaker.
Pick up; "What the !@#%* is this!"
A projectile/railgun like weapon, more effective then the plasmagun but it also has a variable damage. The thing with this weapon is that it hurts pure demons a lot, tech demons some, and demon humans very little. This thing would be useless against former humans but very powerfull against barons and archviles and like wise.
Uses cell as ammo.
Origin; Doom64
The Ambassador
Pick up; "There are two sorts of men in this world: the sort with blood pouring out of a hole in their heads, and the sort holding the Ambassador."
Well then, another pistol unique. This weapon here is a very accurate, yet has a rather slow rate of fire. Bit similiar to trigun but with even more accuracy and possibly slightly more damage.
This gun can also be special reloaded for an aimed shot that is way slower but deals way more damage if it hits.
Origin; Team Fortress 2