If you've watched the original movies recently, chances are you were watching the Special Editions — the versions with updated visual effects.
Lucky for us, Chris Gould of DVD Active has cataloged all of George Lucas's changes in visual and audio effects since the films' theatrical releases in 1977, 1980, and 1983.
Stars Wars: A New Hope
It starts right away when the Lucasfilm Ltd. logo gets all fancified:
R2-D2 wanders down an even lonelier canyon on Tatooine:
Stormtroopers discover C-3PO and R2-D2’s presence aboard the escape pod:
The Tatooine sunset is redder:
Obi-Wan Kenobi's home becomes far more grandiose:
The Wolfman in the cantina transforms into an alien:
By far the most controversial change in the Special Editions, Greedo now shoots first/roughly at the same time as Han Solo:
Obi-Wan Kenobi deactivates the tractor beam to allow the Millennium Falcon to escape from the Death Star, and the language changes from English to Aurebesh:
Han Solo confronting a few Stormtroopers turns into Han running into a hangar full of troops:
The destruction of the Death Star gets more epic:
This scene with Han Solo and Jabba the Hutt was added into the 1997 Special Edition release. In the 2004 update, Jabba was redesigned to look more like he did in The Phantom Menace.
The Empire Strikes Back
This shot is from the perspective of Luke Skywalker, who's just been attacked by the less vague-looking Wampa:
The original hologram of the Emperor was played by an old lady with chimpanzee eyes.
Cloud City got a major update:
Darth's lightsaber turns pink:
The Special Edition release added a scream to Luke's previously silent fall after his duel with Darth Vader.
Return of the Jedi
Right before Chewbacca grabs Han’s ankles, Han hangs off the skiff by his toes alone in the original:
Here is a probably unnecessary visual effect change of Han Solo's face while he is freed from his carbonite prison:
For the Special Edition, Lucas once again added more audio at a climactic moment: when Vader decides to sacrifice himself to save his son.
The spirits of Obi-Wan Kenobi, Yoda, and Anakin Skywalker watch Luke and the Rebels celebrate their victory over the Empire.