Star Wars: Empire at War had unique scenario that didn't appeared in base game.
Star Wars: Empire at War: Forces of Corruption had unique tutorial extension, that was removed in final product.
Freespace 2 had unique 2 missions. First one was quite epic, in a nebula. I remember barely destroying enemy
cruiser, when the second one jumped shortly after. On a collision cours. And my engines / thrusters were disabled.
Yeah, that demo mission was quite epic. I was little dissapointed, that they didn't appeared in the full game.
Homeworld OEM - not a DEMO per se, but it can be traced. It contain special fifth mission,where you have to destroy two
separate Turanic Raiders fleets, instead of one strong Taiidan fleet. Only 5 missions, but skirmish was OK i think.