Outcast: LN-DUO 500, mine/grenade launcher.

What if there's a broomstick, but it's not a weapon? (In Might and Magic 2, you can get a broom, but it's an item that casts Fly when used.)

Boomstick, not broom. If you've played Duke Nukem 3D or Ion Fury, you would've noticed.

You forgot mentioning EVIL DEAD. :)

A few more:

Dynamite from Blood - Simple, effective. Can work on a time delay and bounces like a grenade. Goes boom. You could ask for more from it, but what else could you truly want?

The god mode power-up from Rise of the Triad - Okay, technically not a weapon and it only lasts for half a minute but you still get to point at fools and watch as they get wiped from existence by the ball of god power you fire at them.

Penumbra Overture has a broom. It is meant to be used to get hard-to-reach items, but you can also use it to hit dogs.

Usually it just makes them more angry.

Quite a lot. Here is what pops up immediately:

Duke Nukem 3D
- Devastator. Pretty overpowered and unending destruction ensues (as long as you have ammo).

Doom (the old ones)
- BFG 9000. Because BFG 9000.
- Plasma Gun. I love how loud this gun is. Gives it quite a powerful feel and is pretty much the best DPS weapon besides BFG with insane pain-lock potential.
- Rocket Launcher. Every single rocket is pretty devastating and you can spew them out very fast. Great for long range sniping of crowds where BFG would be ineffective.

Unreal series
- Redeemer. From the name to the fact that even you can now manually guide your personal nuclear device anywhere you would like :P.
- Flak Cannon. Always loved this gun. Absolutely devastating and the 2 modes of fire basically mean it never feels useless in any situation.
- Ripper (Razorjack). Bounces around corners and there is something satisfying about firing supercharged circular saws at people. Kind of confused why it was the one gun that they decided to take out in the later instalments as it was one of the more unique ones.

Serious Sam
- XL2 Lasergun. 4 barrels of rapid fire laser action. What's not to like?

Dark Forces and Jedi Knight series
- Disruptor Rifle. Immensely satisfying to just evaporate and erase an enemy from existence.
- Stouker concussion rifle. One of my favorite weapons from the Star Wars universe. Either a big blast for clearing mobs of enemies or concentrated beam for single tough opponents.

Blood
- Napalm Launcher. One of the most destructive weapons I've seen in any game. Insane rate of fire and absolute beast of a weapon. When used against crowds, fire, and body parts are just flying everywhere.
- Dynamite. Probably the best thrown weapon in any FPS I've played. Ripping cultists to shreds by tossing a dynamite at them never gets old.

Fallout
14mm pistol. Because it is so insanely loud and everybody loves a good BOOOM. Note that the sound is changed in the restoration patch (for some reason) to a standard pistol sound which I hate.

Return to Castle Wolfenstein
- Venom. Basically a perfect hand-held minigun. Love firing that thing and just going BRRRRRRRRRRRRRRRT on anything that crosses my path.
- Snooper Rifle. Because holy shit that gun is HUUUGE.
- FG42 Paratroop Rifle. Just love the rhythm of this weapon. And the reload animation is addicting for some reason.

Carmageddon
- Pedestrian Electro-bastard Ray. Nothing makes me beeline towards the nearest population center faster than finding this puppy :P.

I would like to humbly suggest the electric gun(forget the name) and the flare gun....nothing more fun than watching enemies run around on fire or be shocked into paralysis.

That sounds like a lot of fun. Almost makes me wish I had a Playstation (or any other console) to try it out.

Multiclassing in PoR:RoMD can lead to absolutely insane results, particularly if you solo or duo the game (which allows you to reach the level cap of 32, and every feat you could possibly want).

Two-handed weapons, for example, means you have to forego a shield; but monks have a higher AC without one, since they're allowed to add their both their dexterity and wisdom bonuses to their AC. Add to that a few levels of sorcerer (for AC buffs like Mage Armor and Shield), fighter (for Power Attack and Cleave), paladin (for the charisma bonus to saving throws) and rogue (for Mobility and Uncanny Dodge), plus all the stat- and skill-boosting items available throughout the game, and by the time you get to the end, you don't face the final boss; the final boss faces YOU.

Tesla Gun is extremely overpowered but otherwise did not leave a lasting impression. Just a gun that goes pew pew pew but shoots bolts of electricity. And the secondary fire, while flashy, is a waste of ammo.

Flare Gun is good but has nothing on Napalm Launcher. It rips apart everything and whatever survives the explosions is set on fire. From a gameplay perspective, Flare Gun is also useless on a lot of enemies. Dealing almost only pure fire damage, the fire dogs, Phantasms and grey Gargoyles are immune to it. Napalm deals fire/physical damage so it can be used against pretty much anything.

Another comes to mind:

Hexen: The Wraithverge

And overpowered is a bad thing because? ;)

The flare gun is still fun for the lesser enemies, and with the napalm launcher they usually don't run around for a bit and just get blown up...which is nice as well in a way, but I still also like the flare gun for some enemies. :)

Because it makes every encounter trivial. The gun has everything. From the ludicrous amount of ammo in every pickup to the rate of fire and damage per shot. It melts everything ridiculously fast. If the ammo was scarce at least, then that would be something but it actually has no downsides. The only one I can think of is that is is useless against Phantasms but that is not a downside per se because most of the weapons are actually useless against them.

They still can with the Napalm Launcher although is is not that often. It still deals fire damage but for enemies to go into the fire death animation, they have to get almost killed by the explosion so that the fire damage finishes them off. When you shoot into a crowd, one or 2 enemies usually die this way. Brown Gargoyles can sometimes die with the fire animation after a direct hit.

I pretty much use the flare gun only for the alt fire. But nothing against the gun itself, just that there are usually better and mainly, safer options (playing on Extra Crispy).

Note that you can't get that much damage in other versions.

On NES, the level cap is only 50, and the Giant's Glove and TMPR spell don't work, limiting your damage output. (I think you can still break 2,000 damage, and the final boss only has 2,000 HP on NES, however.) The PSX version on Normal mode has the same level cap of 50; the Giant's Glove and Steel (what TMPR is called in this version) actually do work, but critical hits get less of a damage bonus I believe. I believe the WSC version is like the PSX version but without the option of Easy mode.

On the MSX version, Monks are broken; they don't fight better unarmed than other classes, which makes the class completely useless. (Then again, this version was never released outside of Japan.)

On GBA and later, the Monk damage formula has changed. The new formula, which uses Stamina instead of using the character's level, results in a Monk that's stronger unarmed at low levels (to the point where nunchucks are pointhess in these versions), but the maximum damage you can deal is not quite as high (the 2 most powerful bosses the GBA version added can survive it, for example); it's still enough to potentially kill the final boss, who has 20,000 HP now, in one hit with some preparation. Interestingly enough, on GBA if you manage to deal over 32,767 damage, the damage display will be glitched.

(Speaking of damage glitches, apparently in Final Fantasy 7 it is possible to deal negative damage if you use Barrett's or Vincent's ultimate weapon and power it up enough; due to the way the game handles it, it is possible to kill Emerald Weapon (but not Ruby Weapon) in one hit despite the 9999 damage cap.)