A short guide:
A nice little melee weapon, but not something you'll really want to use as a matter of course. Can be used to jump to some insane heights, but this requires skill to avoid killing yourself. You need to hold down primary fire for a bit to charge it up, then run towards your enemy and release. If you're lucky, you'll disembowel them and see gibs fly everywhere. If you're unlucky, you'll be very close to someone with a Flak Cannon...or a charged up Impact Hammer. Oops.
The Enforcer is your default weapon. It's a pistol, basically, with two modes of fire. Primary fire is just a slow, but absurdly accurate, single bullet firing mode. Secondary fire is a little faster but don't bother using it at a distance because it's so goddamn inaccurate. If you kill someone who's holding an Enforcer, they will drop it and you can pick it up, so you have two at once. Nice. I've fragged quite often with this, but really it's only for use when desperate.
Shoots globs of green sludge, which stick to surfaces for a spell and then explode, making a lovely booby trap. Aimed directly at people, it can cause a hell of a lot of damage. Primary fire shoots rapid gobs of goo everywhere, while secondary fire can be held down and then released for a massive glob of death. Not really good as a primary weapon, but it has its uses.
Directly descended from the ASMD in the original Unreal, this shoots an instant bolt of electricity out, which is fairly damaging to whatever it hits. Secondary fire shoots a slow moving ball of energy which causes a fair bit of damage. If you shoot the ball of secondary fire with the primary fire, it will give off a fairly large explosion (although it requires some skill to kill with this). It's superseded by the Pulse Gun.
The pulse gun is a beefed up shock rifle. Primary fire rapidly shoots balls of energy, which can inflict mortal damage, while secondary fire, when held down, shoots a long lightning bolt. Track an opponent with this to frag them silly. A nice weapon, but watch your ammo.
Basically a very, very fast Enforcer, with the same changes in speed and accuracy on secondary fire. Good if you want to go on a murderous rampage.
The Ripper shoots out razor blades which ricochet off of surfaces and sink into your opponent. Before you get ideas, it's not very easy to just put out a load of blades and obliterate people: it needs a little bit of precision. Aiming for the head will take it clean off-instant frag! Secondary fire shoots the same blades, but they do not ricochet on impact and instead explode.
My second favourite weapon, the Flak Cannon can be used as both a melee and long range weapon. Primary fire shoots out lots of pieces of metal shrapnel, which hurt opponents quite badly. Secondary fire shoots out a shell, which explodes in a similar manner. Use primary fire up close, use secondary from far away (just aim it well).
Aim this well and things will die fairly rapidly. Primary fire shoots out fast moving rockets, secondary lets out small bombs which bounce along the floor and then explode after a short time. On both modes, holding down the button will load more rockets into the launcher and allow you to let loose more than one. Holding down primary fire, then holding down secondary fire simultaneously will launch six rockets in a tight circle. Curiously, aiming for the feet with primary fire does a lot of damage.
This is a great weapon which can kill in a single shot if you aim well. Primary fire just fires a bullet, secondary fire allows you to zoom in to your target. Find a nice ledge and have fun: I've fragged 79 people in a few minutes like this before (although most players frown upon such behaviour). Aim for the head and get an instant frag.
Weapons that aren't really weapons
Oh. Dear. The Redeemer shoots out a slow moving missile, which on impact detonates like a nuclear weapon, creating a deadly shockwave. Aim it well. Secondary fire allows you to use the mouse to guide the rocket from the rocket's view. In this mode, opponents (and you) will have a small red target over them and you can detonate by pressing the primary fire button. Fun fun fun.
The translocator allows you to teleport to places you couldn't otherwise reach. Primary fire shoots out a small beacon, and secondary fire teleports you to the location of said beacon. Shoot the beacon underneath an opponent and then teleport to kill (telefrag) them. If someone shoots your beacon, and you subsequently teleport to it, you will die. Whoops.
Only rarely found. Primary fire plunges it forward, secondary swipes it sideways in a slicing action.