The following is a record of what Lion Voltron can do weapon-wise (click on thumbnails to view larger image). Since I haven't scoured all the episodes, this may not be a complete list. If you know of something that's missing here, feel free to email me.
Of course, the individual Lions have their own specialized weapons, of which I may chronicle separately at some point. This also doesn't include any of Voltron's hand-to-hand types of skills such as kicks, holds and so on he can do without firing a shot. This is strictly the weapons the lions can use when combined as Voltron.

Spinning Laser Blades
Description: Lion Voltron can form a spiky disc, which he can throw at an ememy to do considerable damage. The blade is powerful enough to destroy ships and do considerable debilitating damage to a robeast.

Lion Head Attack
Description: Introduced mid-way through the series, Voltron pulled this trick out of his hat for the first time, I believe, when battling the speedy Carp in "Lotor's New Hitman", and used it here and there afterwards. The attack involves all four of Voltron's appendages to lose their heads and go flying at the enemy (although I do recall a few eps where I believe only the lion heads on the arms blast off). The advantage is surprise and speed, and the attack will leave the enemy with more holes tore through than before - pretty debilitating!

Lion Torches
Description: The Lion torches are more than just fire-breathing appendages. Actually, the heads of the Lions that comprise Voltron's arms and legs can fire any number of substances. Sometimes they fire elements that match what each Lion is supposed to represent - fire, wind, water, and so on. Sometimes it's all wind, all fire, etc. Depends on what old Voltry needs to get done. This attack can weaken a robeast but I don't think it's ever finished one off.

Sting Ray Missiles
Description: Usually firing out of the mouths of the Blue and Yellow Lions, these missiles prime an enemy for the inevitable end, though they rarely finish off a significant foe.

Ion Darts
Description: These are beams with tiny darts in them that Voltron shoots at an enemy. They originate from part of his head, and really don't do that much damage.

Blazing Sword
Description: This is Voltron's ultimate weapon. LV creates the Sword by bringing his two "hands" together, then parting them with the Sword materializing in between. It's not your ordinary sword, for it's pretty powerful and can slice through metal like it's tin foil. This is the weapon that almost always decisively ends any battle - makes you wonder why Volty bothers with the other weapons ;)

*BS note: In "Pidge's Home Planet", the blazing sword destroyed a robeast remotely, which was rather unsual. Using something called Star Fire, a four-pointed "star" came out of the Sword and hit the robeast, slicing it in quarters and destroying it. See images below.

Eye Beams
Description: Just like in Vehicle Voltron, Lion Voltron can shoot destructive rays of energy out of his eyes at an enemy. While it packs a good wallop, it rarely dispatches major aggressors such as a robeast.

Electro-Force Cross
Description: This cross emits from the one on the Arusian seal, which adorns Voltron's chest. The cross in this form flies out and strikes its victims, usually in the head. It rarely does massive visible damage, but it seems to stun or paralyze the target, buying Voltron time to get out the serious toys, such as blazing sword.

Lion Lariat
Description: This rare item seems to magically form out of Voltron's body, and it actually functions more like a pair of nun-chucks than a lariat. This was used in the episode "The Buried Castle", and Voltron used the items to deflect an assault from incoming projectiles. It seems to be more of a defensive weapon than one of attack.

Lion Saber
Description: Lion Voltron reaches behind his head and pulls out two spears, which he then puts together to form a double-ended saber. He throws this weapon like a javelin to disable an enemy, though it usually isn't powerful enough to take out a robeast. This is a similar weapon to VV's solar combat spears.

Argon Force Field
Description: Extremely rare for Voltron to use, he employed this trick on the Avok robeast during "My Brother Is A Robeast". Basically, it's this ball of energy he releases, which encases Voltron in a sort of force field AND it traps his enemy in a similar field effecting a sort of stalemate.