Okay, it was a long time in coming, but here it is at last.
(Note: These are presented in no particular order)
1. The Flamer
I consider this to be one of the "Staple" weapons of the Imperial Guard. It's cheap, can put out a lot of wounds, and doesn't need to roll to hit. It's great for toasting Orks, Nids, IG, and Eldar, and has the bonus of ignoring cover. It's range is a bit of a handicap, as are the Str and AP of the weapon.
Tips for use: I like putting these in PCS squads, and using them for counter assault and objective grabbing. 4 Templates will make even MEQs sweat those armour saves, and will roast just about any infantry unit. They're also a good staple in mobile platoons, flankers, etc. Some people don't like putting them with Vets (because it's a waste of BS 4), but Vets in Carapace with 3 Flamers and a bunch of Shotguns are a very nasty counter charge unit. Some people also put them in line squads when they know they're going to get charged (like when playing Orks). It can be effective, but it limits the amount of ranged shots you put out.
2. Grenade Launcher
This is the other "Staple" weapon. This, and the Flamer, should really be the only weapon your basic infantry should ever really have. Anything else is too expensive or dangerous to be used properly by line squads. Anyway, the GL is a good multi purpose weapon to give to your boys. It has good range for an SW, and the Krak round will do ugly things to most infantry and light vehicles. The Frag round is worse, as it's essentially a lasgun shot that uses the small blast template, and will therefore struggle to bring down anything with a toughness of 4 or better. Even against IG, it's not too great, as it only wounds on a 4+.
Tips for use: I put these in my Gunline squads, just to give them a little ranged punch. A PCS rolling 4 GLs will mess up small squads of Eldar and Tau pretty badly, and can usually pick off a Marine or two. That said, I don't recommend them fot Command Squads, since other weapons are better there. If you're having trouble with Rhinos or Trukks, take a Vet Squad with an Autocannon and 3 GLs. Three str 6 shots and two Str 7 shots, all hitting on a 3+, will end most light vehicles.
If you want to kill armour, Termies, high toughness creatures, or SCs, use this. It's reasonably priced, but for only one shot, you want to put them in a squad that can take several, and preferably hit on 3+. It has very short range as well, so you have to be close. That said, if it hits something, it almost always going to kill it.
Tips for use: I always give it to Vets with shotguns. If I know I'm facing deepstriking Termies, Drop Pods, or Battlesuits, I keep them in reserve. Then, when the target shows up, I march my Vets on, have them shoot the crap out of the target, and then charge. It's usually enough to finish them off, or at least tie them down. Giving Vets Meltas and packing them into Valkryies also tends to work out nicely, especially if you give them Shotguns and "Demolitions". That combo will kill just about any tank you point them at. It's also a good combo for Stormies, allowing them to pop up somewhere and shoot a tank to death.
4. Plasma Gun
Very much a staple for fighting MEQs, Plasma does ugly things to anybody rolling around in Power Armour, and can mess up Rhinos and Trukks as well. It also puts out a lot of shots, and has good range. The downside is that it's expensive. Oh, and it might blow up in your face.
Tips for use: The only army I never use Plasma against is IG, just because there's no point. Every other army has a 3+ or 2+ armour save floating around in their codex. Some people like to take Plasma with Vets or Stormies, but I find that to be a waste. For 20pts less then Plasma Vets in Carapace (and you ALWAYS want to have a better save then 5+), you can give your CCS 3 Plasma guns and a Medic, which means that in the case of an overheat, you have a 5+ save AND a 4+ FNP save. Stormies are okay with plasma, but remember; Plasma is Rapid Fire, so the Stormies can't use their Hotshot pistols and then charge.
5. Heavy Flamer
I would consider this to be a "maybe" weapon. It's better then it's little brother, the Flamer, and can punch through 4+ armour and wound most basic infantry on a 2+. It suffers from the same range drawback as the Flamer, though, and is VERY expensive for an SW.
Tips for use: Putting them with Counter Charge Vets and PCS' are really the only good place for them, and you might as well just save the points and go with a normal flamer instead. 3-4 Templates will burn up a lot, even if they're mid Str. I would say skip it, unless you really need it or you can't spend the points elsewhere.
6. Sniper Rifles
Ooooh boy, here we go. Now, as some of you from the IGMB know, I don't like Sniper Rifles. I may have even called them "A Swirling Vortex of Pure Suck" on more then one occasion. So, I'll try to lay out the basic stats, then explain WHY I hate them. They are pretty cheap, and have the best range of any SW. They always wound on a 4+, which can be an advantage if you're taking pot shots at the Nightbringer or other High Toughness enemies. It also makes them horrible for dealing with Str 3 armies, like the IG or Eldar. Also, they have no AP, and have to rely on Rending to punch through armour. They also cause pinning.
Okay. Here is why I hate them and they suck. Say I have a Special Weapon Squad with three Sniper Rifles. I take my shots (*rolls dice*) and get two hits. Now, I roll to wound (*rolls dice a second time*) and get a single wound. The target shall now take an Armour save (We're going to say 4+, because that way we can represent an armour save or a cover save, as most targets have a cover save with the 5th Ed rules), and passes. Assuming he didn't I rolled a Morale check, and passed that. Thus, you just payed about 40-50pts for a unit that did absolutely nothing.
Let's try it again with GLs and krak rounds (Let's assume the target has Toughness 3 or 4, and can therefore be wounded on a 2+). I have 2 hits. I have 2 wounds. Assuming they have a cover save, one failed. Assuming they were relying on Armour, both would have been dead. Thus, you can choose between a unit that will wound (and very probably kill) it's target if it hits, as opposed to a weapon that may not wound it's target, and will give them an armour save 2/3rds of the time.
Sniper Rifles are horrible against T 3 and 4. Against T3, EVERY SINGLE SW wounds on at least a 3+, except for Sniper Rifles. Against T4, the Sniper Rifle and the Flamer wound on 4+, and everything else does better. Even here, the Flamer is better, because it can dish out multiple wounds, and doesn't need to roll to hit. Against T5+, go ahead and pay the extra points for Plasma guns or Meltas, because they almost always match or best the 4+ to wound required of the Sniper Rifle, and they ALWAYS deny the target an armour save.
So, yeah, don't take Sniper Rifles.