Below is a link to secret unused missions found in the StarCraft 1.0 disc.
There have been videos of them on Youtube along with an instruction video, but for those who don't have a 1.0 disc, i have uploaded this ZIP file containing the 4 unused maps.

Protoss.scm: This is a slightly different version of the first protoss mission, i think some team colors are different, and you can use the whole tech tree, but that's all different I could find. It's been a while since I played the used first protoss mission but i think in this version there's another door in the base, I just don't remember if that was there before.

Terran.scm: This mission i believe takes place between terran missions 7 and 8. You fight zergs. there's a briefing complete with sound. I had to modify triggers so that tassadar and his crew spawn. all I had to do was set a switch at the end of a trigger. but even then there's a bug where duke says that there are more zerg then they can handle after tassadar appears. it's supposed to be the other way around. oh well, cope with it.

Terran2.scm: This was not included as blizzard decided to use "the Big Push", the current mission 8 of terran, instead of it. but it's pretty cool. seems to be ok so far.

Zerg.scm: In the StarCraft CD this was listed as a Zerg tutorial. it didn't have any custom sounds with it, so there's no dialog, and no mission briefing. Very not completed, but that's the way it came. You have 2 drones and a hatchery, but no overlord or minerals. at the other end of the map there's a stray marine that you can kill to win the mission, but there are no objectives listed. I actually had to modify the map to make it playable in single player, by making White player a computer and in a different force. Other than that, the map is identical to the way it came. you can't build any more units then the 2 drones you start with unless you add an over lord and minerals in the map. but your 2 drones can easily take out the marine.

Download Secret SC missions