There are quite a few upgradeable items scattered throughout the game world; these will almost always wind up being better than stock items of the same type. When you possess one of these items, you can take it to a workbench (there's one on the Ebon Hawk), and modify it with spare parts to make it better...faster...stronger. Spare parts can be taken in and out of weapons, so if you find something that you think might replace one of your current items, you can strip the old item of its parts and put them into the new item to see how they compare.
Many of these items are available for purchase from various vendors at exorbitant prices. We've taken the liberty of purchasing as many of these as we could and upgrading them completely to give you a before-and-after picture of what they'll look like, so that you don't have to save and reload if you dislike an item. This shouldn't be considered an authoritative listing; there are sure to be other items out there.
Note that there are no unique lightsabers; each lightsaber you find will have the same beginning stats as all the rest.
Carth's Blaster
Proficiency: Blaster Rifle
Found: Carth is initially equipped with this weapon.
Before upgrades: Energy: 2-7
After upgrades: Energy: 4-9, +2 Attack
Bendak's Blaster
Proficiency: Blaster Rifle
Found: After defeating Bendak Starkiller in the Duel Ring on Tarsis.
Before upgrades: Energy: 2-7, +1 Attack
After upgrades: Energy: 5-10, +3 Attack
Zaalbar's Bowcaster
Proficiency: Blaster Rifle
Found: In a locked footlocker near where Zaalbar is imprisoned in the sewers on Tarsis.
Before upgrades: Energy: 1-10, +1 Attack
After upgrades: Energy: 3-12, +3 Attack, Keen (double critical range)
Ordo's Repeating Blaster
Proficiency: Blaster Rifle
Found: Canderous is equipped with this weapon when he joins your party.
Before upgrades: Energy: 2-11
After upgrades: Energy: 5-14, +1 Attack, Keen (double critical range)
Jamoh Hogra's Carbine
Proficiency: Blaster Rifle
Found: Bought from merchant on Tatooine.
Before upgrades: Energy: 3-10, +1-4 Physical, Attack +2
After upgrades: Energy: 5-15, +2-8 Physical, Attack +5
Jurgen Kalta's Assault Rifle
Proficiency: Blaster Rifle
Found: Bought from Fazza Utral in the Hunter's Lodge on Tatooine.
Before upgrades: Energy: 1-8, +1-6 vs. Droid, Attack +3
After upgrades: Energy: 2-12, +1-6 vs. Droid, +1-4 Ion, +1-4 Physical, Attack +5
Cassus Fett's Heavy Pistol
Proficiency: Blaster Pistol
Found: Bought from B'ree on Korriban (10,000 credits).
Before upgrades: Energy: 4-11, Attack +3, 25% chance of stunning target for 6 seconds on hit
After upgrades: Energy: 6-19, Attack +5, 25% chance of stunning target for 6 seconds on hit, Keen (double critical range)