Superman 2: The Richard Donner Cut. I grew up on the tv version of the film (also known as the ABC cut here in America), and a lot of the DC scenes are in it. When the Donner Cut was released and wasn't the Holy Grail I expected it to be, I somehow came across this site some months later. Growing up on the tv version, I remember buying the movie at the store again when I was 12 and noticed a good 20 minutes of the movie was missing. I found out about the tv versions additions in 2002 and bought the ABC cut on vhs online for 25 dollars (ouch). I hated paying that much, but I wanted to see the film as I remembered it. I also bought the 188 minute version of Superman 1 with it because I was curious to see that too. Another 20 bucks.
My interest in editing films, or just knowing about alternate takes goes back much further. As a kid, I'd always notice when movie trailers would have scenes that weren't in the films, or different takes (same dialogue but delivered different). My earliest memory is the scene from Bambi when the older deer keeps telling him to get up. "Get Up Bambi" he'd say. But when I saw this clip in a Disney feature that showed clips from all the different Disney films, the deer's voice was different than the one in the actual film.
Also my dad's friend who is a huge movie buff (he loves fan edits as well) used to get us bootlegs of movies while still in theaters, and back then the bootlegs were usually workprints of the films. To this day I still still get weirded out when I watch Space Jam or Men In Black because the workprints that I watched a million times were slightly different in effects, dialogue, and soundtracks (and in some cases, better). In MIB's workprint when Smith catches up to the punk/alien he chases at the beginning, instead of having his gun drawn when he opens the door, the alternate take showed Smith leaning forward, out of breath and says "Hey...what's up?" very breathlessly, while cocking his gun. Much better/funnier take IMO. In Space Jam's workprint they play the beginning of U2's "Where the Streets Have No Name" as Jordan makes the winning dunk. I just thought the song fit perfectly (I have a thing for music videos as well. Nothing excites me more than the perfect syncronization of audio and visual. It's truly magic to me).
I can't remember when I heard about the producers cut of Halloween 6, but I can remember looking up websites that would have rare, out of print stuff regarding films, be it workprint versions, director's cut, or anything else not publicly released during the early days of my having a computer, like the early 2000's.