Alternate Reality was one of the more promising Starcraft forumer campaigns that no one seriously expected to get finished (a genre with a fair bit of competition, understand). It was created by Admiral Styles and His All-Starr Band. The first episode was released in April 2002, and the campaign was cancelled in March 2003.
You can download Episode I, the only episode to be completed and released, and find more details about the campaign on the Alternate Reality web site.
There were some fairly neat unfinished missions from Episode II that unfortunately never made it to public consumption. Here's the unfinished third mission, in which the gang heads out to the Holiday Planet.