U.S. Marines wear swords with their dress uniforms for the sake of looking cool (and honoring tradition), but they don’t carry them into combat. If for some reason you need to slash or stab something, you can use your bayonet.
Now that makes sense. Provided, as noted above, that the situation allows you the luxury of "close-combat, i.e., your opponent does not have a weapon of greater range.
That makes sense too. But, if I were a Jedi, I would never think of going into any situation armed only with a lightsaber. Remember, when Luke boarded the Death Star to rescue Leia, he got a lot more use out of his blaster than his lightsaber. Whereas we get the impression that old-school Jedi like Qui-Gon Jinn and young Obi-Wan Kenobi would not even know how to aim or fire a blaster.
A few entries:
The abdiatic bomb (from Killing Star): turns heat into mass - instant freeze.
The microscopic “kill anyone instantly, without a trace” gizmo Worsel dreamed up for Kimball Kinnison.
Gray goo.
And a shameless plug: the Shiva: The Shiva (SWT29.1) is a C-fractional weapon. It has a 2,000 ton AM warhead, subdivided into 20,000 sub-munitions, each weighing 100 kg. It is launched from the outer reaches of the system and accelerates to about 0.9 C. Just before impact, the warhead splits and the sub-munitions spread wide enough to cover the entire facing hemisphere of the targeted planet.
Well, yeah, duh. But the Jedi do have the Force to call on, which is why it doesn’t matter that the lightsaber isn’t a long-range weapon.
Like hell. The guy with the blaster would never come close to even touching a Jedi. Lightsabers can parry blaster bolts, and a talented enough Jedi can bounce them right back where they came from. A stormtrooper could get off maybe three shots before his first shot came back and pegged him 'tween the eyes.
Also, keep in mind that when Luke went to rescue Leia from the Death Star, he’d had maybe two weeks of Jedi training, max. But by Return of the Jedi, he’s stopped wearing a blaster altogether, IIRC.
In with you on the Klingon bat’leh, though. Stupid looking, impractical weapon. The daggers are a bit more plausible, as there is a real-world analogue. Can’t recall the name of it off the top of my head, but there was a type of dagger that had the two spring-loaded extensions like the Klingon knife. Except that it was specifically designed as a defensive weapon to counter the rapier. You’d catch the blade between the two extensions, then twist your arm and either disarm the other guy or snap his blade in two. Don’t really see that working against at bat’leh…
The Pulse rifles in Aliens were pretty cool. 90 rounds in a little magazine and an ammo counter? Nice.
The Sister Ray in FF7 was sweet, even if pretty damn useless.
A couple of thoughts:
The Shrike, if you count it as a “weapon”.
The Hammer’s Slammers’ hovertanks/communication/IFF/aiming equipment.
The Gauss rifle and super sledgehammer from Fallout 2. God, I loved those things.
Well, it does move outside time. Tough to beat that.
I always thought the ferengi electric whips were cool (if impractical, what do you do in close quarters?)
Borg Nanoprobes, eating you up from the inside is neat
R2D2’s ewok roaster was kinda fun (Used for a good purpose)
The Batleth!
Can you tell I’m a star trek fan?
Ooh! Another Trek weapon.
Annorax’s weapon that erased civilizations from time. This was in a few Voyager episodes. It literally erased species from history.
Thats a cool weapon.
On a related note, I’m going to nominate the Skull Guns from
The Diamond Age. A tiny, undetectable cannon surgically implanted in your forehead that fires minute nanotechnological ammunition. I mean, come on, it’s a gun that goes in your head .
I’m surprised we’ve got this far with no mention of Ice-9. Of course, there are some disadvantages to using it, say, in a combat situation.
Just make sure that when you cast Ice-9, your opponet is in a parallel universe, or stuck in a bag of holding, and you won’t have a problem.
Ice-9 is fine in a combat situation, as long as you’ve got a bag of holding. (Preferably one not stuffed with more riches than actually exist.)
And on preview, I see that RandomLetters has beaten me to the punch. Curses! Foiled again!
My favorite is the Shadow planet killer from Babylon 5. Gotta love a show where you have to specify which planet killer you’re talking about. 
Either that or the n-space planetoids in Doc Smith’s Lensman series. Basically, you put a big ass engine on a suitably large chunk of rock, switch it over to n-space, where the laws of physics no longer apply, accelerate it to hundreds of multiples of c, and then switch it back to normal space, aimed at your target. Target then dies in a spectacularly non-Euclidian fashion.
That’s the one!
You are forgetting that Luke is basically a novice Jedi. He did not get the years of training that most Jedi’s get so don’t base the weapon on his skill with it. I hate to say this but look at what the Jedi in the first two films can do.
Heck look at Vader in ESB. Han, who is pretty good with a blaster, draws and fires on Vader to no effect. Vader uses the force to CATCH the balster fire. He doesn’t even have to deflect it.
For me the coolest weapons are the phaser from TOS, the light saber, and that gun from the movie Looker that made hot chicks freeze.
Whaddaya mean, “plug”? Got one for sale?