Teebu's Blazing Longsword

Teebu's Blazing Longsword is a one-handed sword with a fire damage proc.

Source
This blade was most commonly dropped during Gates of Ahn'Qiraj event due to the massive amount of 61+ elite mobs that spawned during the event, but it was also known to drop several times in Molten Core. It has also dropped off of Baron Rivendare in Stratholme, but has only occurred once so far. Lastly, this sword is known to drop from Heavy Junkboxes. According to wowhead, this sword has dropped from junkboxes 6 times out of 54190 attempts, making it a 0,011% drop. It is still obtainable, but it is an incredibly rare drop. It is interesting to note that this sword is pictured on the box for the game, so this sword has been in-game since start.

Trivia

 * This sword is displayed on the inner cover of World of Warcraft retail box worn by a level 60 dwarf warrior named "Hammerfall".
 * This weapon looks like a red lightsaber,an infamous weapon used by the Sith in Star Wars. It has been utilized in this purpose in some machinimas.