Hey guys, I came across a really cool Custom campaign on the arcade a few days ago. It's called Subjection, and it's separated into 8 maps/missions. The author does a fantastic job with cinematics, as well as telling a story. The game play is also fairly varied and some missions introduce unique mechanics. Found this campaign very cool as both a player and a fellow map maker. And perhaps a little bit of jealousy of the awesome skills :)
If you enjoy single player mission maps, or just a good story, you should definitely check it out. There's multiple difficulties for those who want a challenge, and those who want to take it easy.