Long story short: here's 30 minutes of footage from Return of the Jedi that didn't make the cut. It's tons of Yoda and Luke stuff on Dagobah (plus some lightsaber test footage), and includes dialogue that would have changed the film somewhat (most notably at the 5:10 mark in this video).
The slightly longer story: Star Wars super-fan Charlie Owen bought a "rare" Star Wars LaserDisc on eBay. And rare it was! That LaserDisc turned out to be full of unedited footage from Jedi, and it was created to demo an early video editing system called the EditDroid. Lucasfilm created the EditDroid in 1983 as a way to edit analogue video using digital technology. This disc was used to demo the EditDroid at a Las Vegas convention in 1984. Nearly 30 years later, boom, it turned up on eBay and Owen began posting clips to Facebook. Now he's through with the clips, and has posted the entire 30-minute disc on YouTube.
If you must pick just one segment to watch (the video does get repetitive after a while), check the reveal at 5:10. Remember, many Bothans died to bring us this information. Oh yeah, and there's no audio in the first scene on the disc.
Tangentially related: Mr. Wizard Explains the LaserDisc.